How to Set the Default App for DMG Files on Mac

Out of the box, macOS hands every .dmg file to a built-in utility called DiskImageMounter. Its one job is to mount the image and open a Finder window — everything after that is your problem. You can point .dmg files at a different app instead, and it takes about fifteen seconds.

The Finder method (works for any file type)

In Finder, right-click (or Control-click) any .dmg file and choose Get Info. Selecting the file and pressing Command-I does the same thing.
In the Info window, expand the Open with section by clicking the small triangle beside it.
Choose your app from the pop-up menu. If it isn't in the list, pick Other… and select it from your Applications folder.
Click Change All… and confirm — without this step you've only changed that one file.

That's it. From now on, double-clicking any .dmg opens it with the app you chose.

Change All only covers the exact file type you clicked. macOS treats .dmg, .iso, .sparsebundle, and .img as separate types, each with its own default. Changing the handler for DMGs leaves the others alone — which is usually what you want.

The one-click method (with EasyDMG)

EasyDMG has a button for this. Open the app from your Applications folder and click Set as Default for DMG Files on the Setup tab. It registers itself with macOS as the handler for .dmg and you never have to touch Get Info.

DiskImageMounter mounts the image and stops. EasyDMG mounts it, runs the same Gatekeeper security check macOS runs, copies the app into your Applications folder, opens it or reveals it in Finder, unmounts the image, and optionally moves the DMG to the Trash. Then it quits — no Dock icon, no menu bar icon, no background process.

Set it once, forget it exists. EasyDMG is free, open source, signed and notarized, and runs on macOS 13 or later.

Download EasyDMG

Opening a single DMG with something else

Changing the default doesn't lock you in. Right-click any DMG, choose Open With, and pick a different app for that one file. Handy when you want to poke around inside an image manually instead of installing it — choose DiskImageMounter and you get the classic behaviour for that file only.

Switching back to DiskImageMounter

Same four steps as above, but choose DiskImageMounter in the Open with menu. It doesn't appear in the list by default, so click Other…, then press Command-Shift-G and paste this path:

/System/Library/CoreServices

Select DiskImageMounter there, then click Change All…. If it appears greyed out, set the Enable: pop-up at the bottom of the file picker to All Applications.

When the setting won't stick

Occasionally macOS keeps opening DMGs with the old app no matter what you pick. This is almost always the Launch Services database — the index macOS keeps of which app handles which file type — getting out of sync. In order of how much you should disturb:

Confirm the app is in /Applications. An app still sitting in Downloads or on a mounted disk image can't reliably register as a handler. Move it, then set the default again.
Open the app once manually. Apps register their supported file types on launch. One launch is often all it needs.
Log out and back in, or restart. Unglamorous, but it resolves most stale-handler cases.
Rebuild the Launch Services database. Last resort. In Terminal:

/System/Library/Frameworks/CoreServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user

That wipes and rebuilds the index of every app on the machine. It's safe, but it takes a minute or two, and the "Open With" menus across your system may look sparse until it finishes. Restart afterwards.