Mac storage basics · Games and virtual machines
How do I move Steam games to an external drive on a Mac?
Short answer: In Steam's Storage settings, add the external drive as a library and move installed games to it. Use a fast APFS SSD and keep it connected while playing.
Steam supports more than one library location. In Steam's Storage settings, add your external drive as a new library, then select an installed game and use the option to move it there. New games can be installed straight to that drive too.
Use a fast SSD formatted as APFS or Mac OS Extended, and keep it connected while you play — Steam shows games on a missing drive as uninstalled until it returns. Load times depend on the drive and connection; a USB-C or Thunderbolt SSD is the sensible choice. More on choosing one: external SSD vs hard drive.