Where does Android Studio save the ProGuard mapping file?

It should be located at build/outputs/proguard/release/mapping.txt in your application module's directory.

In the latest version of ProGuard and Android Studio, the file is located at build/outputs/mapping/release/mapping.txt.

For me they are at 'build/outputs/mapping/release'

I found it cleaner to configure proguard to write the mapping.txt file to a location outside of the build/ directory tree, so that it could be more conveniently checked into version control.

To achieve this, put this in your proguard-rules.pro file:

-printmapping mapping.txt

This will (most likely) place it in the same directory as your proguard-rules.pro file. Ultimately, you probably want to write it to the same directory as your APK file and with an equivalent name (which might include flavor, build type etc).

Note: in my experience, this is not overruled by the proguard template file (which was suggested by a commenter to another answer here).

UPDATE: If you have multiple product flavors, then this is a much better solution: https://stackoverflow.com/a/31116608/444761