Make Unmarked your default Markdown app

Short answer: In Finder, select a .md file, press ⌘I (File → Get Info), open the Open with section, choose Unmarked, then click Change All. Every .md file opens in Unmarked from then on.

macOS picks one app per file type. For Markdown that's usually TextEdit, Xcode, or whatever editor you installed last, so a double-click sends your .md file there instead of to Unmarked. Change All overrides that choice once and Finder keeps it.

Set Unmarked as the default for .md files

1. In Finder, click any .md file once to select it.
2. Choose File → Get Info, or press ⌘I.
3. In the Info window, open the Open with section.
4. From the menu, choose Unmarked.
5. Click Change All…, then confirm.

That's it. Double-click any .md file and it opens in Unmarked.

Change All covers one extension. Unmarked also opens .markdown, .mdown, .mkd, and plain .txt. To route any of those to Unmarked too, repeat the steps on a file of each kind. .md is the only one most setups need.

Open a file in Unmarked just this once

To open one file without changing anything, right-click (or Control-click) it in Finder, hover Open With, and choose Unmarked. It opens this time only, and your default app stays put. Good when you live in another editor and reach for Unmarked now and then.

Why double-clicking sends files to the wrong app

Installing a new editor can quietly grab the .md default, and a macOS update sometimes resets it on its own. That's why a file you expect in Unmarked suddenly opens somewhere else. Change All pins your choice, and Finder honors it until you change it again.