MP4Joiner on Mac: Fix the “Unidentified Developer” Gatekeeper Block

You opened the MP4Tools disk image on your Mac and macOS refused: the app “cannot be opened because it is from an unidentified developer”. That is Gatekeeper, macOS’s check for developer-signed apps, meeting a small open-source program that isn’t signed with an Apple developer certificate. The official MP4Joiner project documents this itself — here is how to open the app the way its own wiki describes, and the honest compatibility limits of a 2019 Mac build.

Why Gatekeeper blocks MP4Tools

By default, macOS only opens apps from the App Store or from developers who sign their software with an Apple-issued certificate. MP4Joiner is free, open-source software distributed through SourceForge without that signing and notarisation process, so Gatekeeper treats it as “unidentified”. The block is about provenance, not a finding that the app is malicious — but you should still only proceed with the genuine file. Our download page links the official SourceForge DMG and publishes its verified SHA-256 hash (f9af7f53e5a9c6c0d576c8c14311f04fb6c39c30919a5594b24c51a64fc7cde0 for MP4Tools-3.8-MacOSX.dmg), which you can check in Terminal with shasum -a 256 before opening anything.

Method 1: right-click and choose Open

  1. Open the DMG and drag MP4Joiner (and MP4Splitter, if you want it) into your Applications folder, as usual.
  2. In Applications, right-click (or Control-click) the MP4Joiner app icon — don’t double-click.
  3. Choose Open from the context menu.
  4. The dialog appears again, but this time with an Open button. Click it. macOS remembers your choice, and the app opens normally from then on.

Method 2: allow it in System Settings

  1. Try to open the app once by double-clicking, and dismiss the block message.
  2. Open System Settings → Privacy & Security (on older macOS: System Preferences → Security & Privacy).
  3. Near the bottom you’ll see a note that MP4Joiner was blocked, with an Open Anyway button. Click it and confirm.
  4. Launch the app again — it now opens, and macOS won’t ask again.

These are the same steps the project’s own wiki page for starting on macOS Catalina describes — the official project keeps a dedicated page for this because unsigned apps have needed this workaround since Catalina tightened Gatekeeper. See our MP4Joiner for Mac page for the full installation walkthrough.

The honest compatibility picture

MP4Tools 3.8 was released in November 2019, and the Mac build is an Intel-era application officially listed for OS X 10.10 and later. Two consequences are worth stating plainly. First, the official project does not claim native Apple Silicon support — on M-series Macs the app depends on Apple’s Intel compatibility layer rather than running natively. Second, the project does not publish support statements for the newest macOS releases, and reviewers have noted that the Mac interface can feel sluggish. For many users it still does its one job — joining MP4 files — perfectly well, and the Gatekeeper steps above are all it takes to start it. But if you need a tool that is actively maintained for current macOS versions, that is a reasonable reason to look at alternatives rather than fight an ageing build.

Once the app opens, the joining workflow is the same as everywhere: see how to join MP4 files. If a join then fails or produces a broken file, the cause is almost never Gatekeeper — work through MP4Joiner not working instead.

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