Where Does Quick Look Store Its Thumbnail Cache?
Quick Look thumbnails cache under a randomized var folder, clearable with qlmanage -r cache.
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Quick Look thumbnails live in a com.apple.QuickLook.thumbnailcache folder buried under /private/var/folders in a randomized two-level path (find yours via $TMPDIR/../C/). A ThumbnailsAgent XPC service fills it every time Finder previews a file. Newer builds also mirror cache under ~/Library/Caches, so check both when hunting size.
The privacy angle is the reason to care. Previewing files from an encrypted volume writes thumbnails to the unencrypted system disk, where forensics can find them after unmount. Disabling generation means sidelining the generator plugins; clearing is just qlmanage away.
Where macOS stores this, by platform
$TMPDIR/../C/com.apple.QuickLook.thumbnailcache
Randomized path per boot user; resolve via $TMPDIR in Terminal. qlmanage -r cache purges; killing ThumbnailsAgent only restarts it.
Frequently asked questions
How do I clear the Quick Look thumbnail cache?
Run qlmanage -r cache in Terminal. It purges the thumbnail store without a reboot. Expect brief delays next time large files preview while thumbnails regenerate.
Do encrypted disk images leak thumbnails here?
Yes, famously. Thumbnails render from mounted encrypted volumes into the cache on the system disk and linger after unmount. Purge with qlmanage and reboot after handling sensitive disk images.
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