diff --git a/packages/base-styles/_mixins.scss b/packages/base-styles/_mixins.scss
index d8b145a2b13dd4..f8ea78b28b3cb3 100644
--- a/packages/base-styles/_mixins.scss
+++ b/packages/base-styles/_mixins.scss
@@ -221,22 +221,6 @@
  * Styles that are reused verbatim in a few places
  */
 
-@mixin snackbar-container() {
-	position: fixed;
-	bottom: 24px;
-	left: 0;
-	right: 0;
-	padding-inline: 16px;
-	box-sizing: border-box;
-	display: flex;
-	flex-direction: column;
-	pointer-events: none;
-
-	.components-snackbar {
-		margin-inline: auto;
-	}
-}
-
 // These are additional styles for all captions, when the theme opts in to block styles.
 @mixin caption-style() {
 	margin-top: 0.5em;
diff --git a/packages/boot/src/components/root/index.tsx b/packages/boot/src/components/root/index.tsx
index 275e8e622d2259..21b7b19d19e4d5 100644
--- a/packages/boot/src/components/root/index.tsx
+++ b/packages/boot/src/components/root/index.tsx
@@ -14,9 +14,10 @@ import {
 	__unstableAnimatePresence as AnimatePresence,
 	Button,
 	SlotFillProvider,
+	privateApis as componentsPrivateApis,
 } from '@wordpress/components';
 import { menu } from '@wordpress/icons';
-import { useState, useEffect } from '@wordpress/element';
+import { createPortal, useState, useEffect } from '@wordpress/element';
 import { __ } from '@wordpress/i18n';
 import { Page } from '@wordpress/admin-ui';
 
@@ -33,6 +34,9 @@ import './style.scss';
 import { UserThemeProvider } from '../user-theme-provider';
 
 const { useLocation, useMatches, Outlet } = unlock( routePrivateApis );
+const { __experimentalGetOverlayLegacySlot: getOverlayLegacySlot } = unlock(
+	componentsPrivateApis
+);
 
 export default function Root() {
 	const matches = useMatches();
@@ -68,7 +72,10 @@ export default function Root() {
 						} ) }
 					>
 						<SavePanel />
-						<SnackbarNotices className="boot-notices__snackbar" />
+						{ createPortal(
+							<SnackbarNotices className="boot-notices__snackbar" />,
+							getOverlayLegacySlot()
+						) }
 						{ isMobileViewport && (
 							<Page.SidebarToggleFill>
 								<Button
diff --git a/packages/boot/src/components/root/single-page.tsx b/packages/boot/src/components/root/single-page.tsx
index 50369a7e18ec8e..e31816d2f80dd8 100644
--- a/packages/boot/src/components/root/single-page.tsx
+++ b/packages/boot/src/components/root/single-page.tsx
@@ -8,7 +8,11 @@ import clsx from 'clsx';
  */
 import { privateApis as routePrivateApis } from '@wordpress/route';
 import { SnackbarNotices } from '@wordpress/notices';
-import { SlotFillProvider } from '@wordpress/components';
+import {
+	SlotFillProvider,
+	privateApis as componentsPrivateApis,
+} from '@wordpress/components';
+import { createPortal } from '@wordpress/element';
 
 /**
  * Internal dependencies
@@ -22,6 +26,9 @@ import useRouteTitle from '../app/use-route-title';
 import { UserThemeProvider } from '../user-theme-provider';
 
 const { useMatches, Outlet } = unlock( routePrivateApis );
+const { __experimentalGetOverlayLegacySlot: getOverlayLegacySlot } = unlock(
+	componentsPrivateApis
+);
 
 /**
  * Root component for single page mode (no sidebar).
@@ -54,7 +61,10 @@ export default function RootSinglePage() {
 						) }
 					>
 						<SavePanel />
-						<SnackbarNotices className="boot-notices__snackbar" />
+						{ createPortal(
+							<SnackbarNotices className="boot-notices__snackbar" />,
+							getOverlayLegacySlot()
+						) }
 						<div className="boot-layout__surfaces">
 							<UserThemeProvider color={ { bg: '#ffffff' } }>
 								<Outlet />
diff --git a/packages/boot/src/style.scss b/packages/boot/src/style.scss
index b4b3569db32f61..1f77b98d884f97 100644
--- a/packages/boot/src/style.scss
+++ b/packages/boot/src/style.scss
@@ -29,6 +29,3 @@
 	}
 }
 
-.boot-layout .boot-notices__snackbar {
-	@include snackbar-container();
-}
diff --git a/packages/components/CHANGELOG.md b/packages/components/CHANGELOG.md
index 3b5950870a7009..c1a9d4d1f9e7f5 100644
--- a/packages/components/CHANGELOG.md
+++ b/packages/components/CHANGELOG.md
@@ -16,6 +16,7 @@
 ### Breaking Changes
 
 -   `ExternalLink`: No longer adds `noreferrer` to the `rel` attribute. `noopener` is still applied. Consumers relying on the previous behavior should pass `rel="noopener noreferrer"` explicitly ([#26968](https://github.com/WordPress/gutenberg/pull/26968)).
+-   The `snackbar-container()` SCSS mixin in `@wordpress/base-styles/mixins` has been removed. Its rules are now baked into `.components-snackbar-list`. Plugins that called `@include snackbar-container()` should target `.components-snackbar-list` directly or inline the equivalent positioning.
 
 ### Internal
 
@@ -23,6 +24,7 @@
 -   `Popover`: Render the fallback container inside the new overlay legacy slot. The popover's per-class z-index is preserved and now stacks relative to the slot.
 -   `Modal`: Portal modals into the overlay legacy slot. Update the aria-helper to walk up from the modal so siblings of the slot wrapper (and outer modals when nested) continue to be aria-hidden.
 -   `Tooltip`, `Menu`, `CustomSelectControl` v2: Pass `portalElement={ getOverlayLegacySlot }` so each Ariakit-backed overlay portals into the overlay legacy slot. Per-overlay z-indexes are unchanged.
+-   `SnackbarList`: Bake the previously-mixin-only positioning (`position: fixed; bottom; padding-inline; pointer-events: none;`) into `.components-snackbar-list` directly so the list self-positions whether portaled or rendered inline.
 -   `NavigableContainer`: Refactor from class component to function component with hooks ([#77171](https://github.com/WordPress/gutenberg/pull/77171)).
 -   `Menu`: Refactor `Menu.Popover` to use Ariakit’s `render` prop, wrapping content in `MenuMotionRoot` (motion styles) and `MenuSurface` (panel layout and `variant` chrome) ([#77460](https://github.com/WordPress/gutenberg/pull/77460)).
 -   Fix types for TypeScript 7.0 ([#77177](https://github.com/WordPress/gutenberg/pull/77177)).
diff --git a/packages/components/src/snackbar/style.scss b/packages/components/src/snackbar/style.scss
index 554ea4b871a0b9..ba77bfb8250823 100644
--- a/packages/components/src/snackbar/style.scss
+++ b/packages/components/src/snackbar/style.scss
@@ -76,15 +76,30 @@
 }
 
 .components-snackbar-list {
-	position: absolute;
+	// Self-positioning at the viewport bottom. Previously these styles came
+	// from the `snackbar-container()` mixin applied to a shell wrapper class
+	// (e.g. `.edit-post-layout__snackbar`). Baking them in lets the list
+	// render correctly whether it is portaled into the overlay legacy slot
+	// or rendered inline by other consumers.
+	position: fixed;
+	bottom: $grid-unit-30;
+	left: 0;
+	right: 0;
 	z-index: z-index(".components-snackbar-list");
+	padding-inline: $grid-unit-20;
 	width: 100%;
 	box-sizing: border-box;
+	display: flex;
+	flex-direction: column;
 
 	// Disable pointer events, so that clicking this area
 	// outside of an individual notice still allows the UI
 	// underneath to be clicked.
 	pointer-events: none;
+
+	.components-snackbar {
+		margin-inline: auto;
+	}
 }
 
 .components-snackbar-list__notice-container {
diff --git a/packages/edit-post/src/components/layout/index.js b/packages/edit-post/src/components/layout/index.js
index 0d2a3b464f973f..ef244ff2b8e9f4 100644
--- a/packages/edit-post/src/components/layout/index.js
+++ b/packages/edit-post/src/components/layout/index.js
@@ -23,6 +23,7 @@ import { getLayoutStyles } from '@wordpress/global-styles-engine';
 import { PluginArea } from '@wordpress/plugins';
 import { __, sprintf } from '@wordpress/i18n';
 import {
+	createPortal,
 	useCallback,
 	useMemo,
 	useId,
@@ -72,7 +73,8 @@ import { useMetaBoxInitialization } from '../meta-boxes/use-meta-box-initializat
 
 const { useCommandContext } = unlock( commandsPrivateApis );
 /** @type {{} & {useDrag: import('@use-gesture/react').useDrag}} */
-const { useDrag } = unlock( componentsPrivateApis );
+const { useDrag, __experimentalGetOverlayLegacySlot: getOverlayLegacySlot } =
+	unlock( componentsPrivateApis );
 const { Editor, FullscreenMode } = unlock( editorPrivateApis );
 const { BlockKeyboardShortcuts } = unlock( blockLibraryPrivateApis );
 const DESIGN_POST_TYPES = [
@@ -619,7 +621,10 @@ function Layout( {
 						<PluginArea onError={ onPluginAreaError } />
 						<PostEditorMoreMenu />
 						{ backButton }
-						<SnackbarNotices className="edit-post-layout__snackbar" />
+						{ createPortal(
+							<SnackbarNotices className="edit-post-layout__snackbar" />,
+							getOverlayLegacySlot()
+						) }
 					</Editor>
 				</div>
 			</ErrorBoundary>
diff --git a/packages/edit-post/src/components/layout/style.scss b/packages/edit-post/src/components/layout/style.scss
index c4d9800a386ee2..ae629bf2847dc4 100644
--- a/packages/edit-post/src/components/layout/style.scss
+++ b/packages/edit-post/src/components/layout/style.scss
@@ -120,6 +120,3 @@
 	isolation: isolate;
 }
 
-.edit-post-layout__snackbar {
-	@include snackbar-container();
-}
diff --git a/packages/edit-site/src/components/layout/index.js b/packages/edit-site/src/components/layout/index.js
index 2b706e1160452f..cad81f482fc485 100644
--- a/packages/edit-site/src/components/layout/index.js
+++ b/packages/edit-site/src/components/layout/index.js
@@ -12,6 +12,7 @@ import {
 	__unstableAnimatePresence as AnimatePresence,
 	__unstableUseNavigateRegions as useNavigateRegions,
 	SlotFillProvider,
+	privateApis as componentsPrivateApis,
 } from '@wordpress/components';
 import {
 	useReducedMotion,
@@ -20,7 +21,7 @@ import {
 	usePrevious,
 } from '@wordpress/compose';
 import { __, sprintf } from '@wordpress/i18n';
-import { useState, useRef, useEffect } from '@wordpress/element';
+import { createPortal, useState, useRef, useEffect } from '@wordpress/element';
 import {
 	UnsavedChangesWarning,
 	ErrorBoundary,
@@ -47,6 +48,9 @@ import SavePanel from '../save-panel';
 
 const { useLocation } = unlock( routerPrivateApis );
 const { useStyle } = unlock( editorPrivateApis );
+const { __experimentalGetOverlayLegacySlot: getOverlayLegacySlot } = unlock(
+	componentsPrivateApis
+);
 
 const ANIMATION_DURATION = 0.3;
 
@@ -156,7 +160,10 @@ function Layout() {
 						</NavigableRegion>
 					) }
 
-					<SnackbarNotices className="edit-site-layout__snackbar" />
+					{ createPortal(
+						<SnackbarNotices className="edit-site-layout__snackbar" />,
+						getOverlayLegacySlot()
+					) }
 
 					{ isMobileViewport && areas.mobile && (
 						<div className="edit-site-layout__mobile">
diff --git a/packages/edit-site/src/components/layout/style.scss b/packages/edit-site/src/components/layout/style.scss
index 540e26aaa41056..4319a7df5f1212 100644
--- a/packages/edit-site/src/components/layout/style.scss
+++ b/packages/edit-site/src/components/layout/style.scss
@@ -273,6 +273,3 @@ html.canvas-mode-edit-transition::view-transition-group(toggle) {
 	}
 }
 
-.edit-site-layout__snackbar {
-	@include snackbar-container();
-}
diff --git a/packages/edit-widgets/src/components/notices/index.js b/packages/edit-widgets/src/components/notices/index.js
index 3605fb0e300600..200ded4596932b 100644
--- a/packages/edit-widgets/src/components/notices/index.js
+++ b/packages/edit-widgets/src/components/notices/index.js
@@ -2,6 +2,17 @@
  * WordPress dependencies
  */
 import { InlineNotices, SnackbarNotices } from '@wordpress/notices';
+import { createPortal } from '@wordpress/element';
+import { privateApis as componentsPrivateApis } from '@wordpress/components';
+
+/**
+ * Internal dependencies
+ */
+import { unlock } from '../../lock-unlock';
+
+const { __experimentalGetOverlayLegacySlot: getOverlayLegacySlot } = unlock(
+	componentsPrivateApis
+);
 
 function Notices() {
 	return (
@@ -10,7 +21,10 @@ function Notices() {
 				pinnedNoticesClassName="edit-widgets-notices__pinned"
 				dismissibleNoticesClassName="edit-widgets-notices__dismissible"
 			/>
-			<SnackbarNotices className="edit-widgets-notices__snackbar" />
+			{ createPortal(
+				<SnackbarNotices className="edit-widgets-notices__snackbar" />,
+				getOverlayLegacySlot()
+			) }
 		</>
 	);
 }
diff --git a/packages/edit-widgets/src/components/notices/style.scss b/packages/edit-widgets/src/components/notices/style.scss
deleted file mode 100644
index 4c7a7bea2d1b4c..00000000000000
--- a/packages/edit-widgets/src/components/notices/style.scss
+++ /dev/null
@@ -1,7 +0,0 @@
-@use "@wordpress/base-styles/mixins" as *;
-@use "@wordpress/base-styles/variables" as *;
-
-.edit-widgets-notices__snackbar {
-	@include snackbar-container();
-}
-
diff --git a/packages/edit-widgets/src/style.scss b/packages/edit-widgets/src/style.scss
index a219bf2ebc17ff..19077821d435ac 100644
--- a/packages/edit-widgets/src/style.scss
+++ b/packages/edit-widgets/src/style.scss
@@ -8,7 +8,6 @@
 @use "./components/header/style.scss" as *;
 @use "./components/keyboard-shortcut-help-modal/style.scss" as *;
 @use "./components/sidebar/style.scss" as *;
-@use "./components/notices/style.scss" as *;
 @use "./components/layout/style.scss" as *;
 @use "./components/more-menu/style.scss" as *;
 @use "./components/welcome-guide/style.scss" as *;
diff --git a/packages/media-editor/src/components/media-editor-modal/style.scss b/packages/media-editor/src/components/media-editor-modal/style.scss
index 58f4e25ae2e558..bb743189a52e91 100644
--- a/packages/media-editor/src/components/media-editor-modal/style.scss
+++ b/packages/media-editor/src/components/media-editor-modal/style.scss
@@ -113,15 +113,10 @@
 	}
 }
 
-// `<SnackbarNotices>` renders `.components-snackbar-list` with
-// `position: absolute; width: 100%`. Portaled to `<body>` it has no
-// positioned ancestor, so it collapses to a 0-size box at the document
-// origin and would also be painted under the modal overlay since both
-// share the default snackbar z-index. `snackbar-container` fixes the
-// list to the viewport bottom; the elevated z-index (registered in
-// base-styles) lifts it above the modal overlay and any popovers that
-// may open inside the modal.
+// `<SnackbarNotices>` renders `.components-snackbar-list`, which now
+// self-positions at the viewport bottom. The elevated z-index here
+// lifts the list above the modal overlay and any popovers that may
+// open inside the modal.
 .media-editor-modal__snackbar {
-	@include snackbar-container();
 	z-index: z-index(".components-snackbar-list.media-editor-modal__snackbar");
 }
diff --git a/packages/media-utils/src/components/media-upload-modal/style.scss b/packages/media-utils/src/components/media-upload-modal/style.scss
index 29f7b67f5988d1..5d9fc9df066601 100644
--- a/packages/media-utils/src/components/media-upload-modal/style.scss
+++ b/packages/media-utils/src/components/media-upload-modal/style.scss
@@ -1,6 +1,5 @@
 @use "@wordpress/base-styles/variables" as *;
 @use "@wordpress/base-styles/colors" as *;
-@use "@wordpress/base-styles/mixins" as *;
 
 .media-upload-modal {
 	.components-modal__header {
@@ -58,7 +57,8 @@
 }
 
 .media-upload-modal__snackbar {
-	@include snackbar-container();
+	// `.components-snackbar-list` self-positions at the viewport bottom;
+	// only the elevated z-index and transform overrides remain here.
 	z-index: 200000;
 
 	// Override the transform and transform-origin applied by the Snackbar component