The following instructions are heavily inspired by /u/uhohohdear. I modified uhohohdear's instructions and then added support for Retina/HiDPI displays and instructions about how to update MTGA.
IMPORTANT UPDATE: As of July 25, 2019, this method no longer works. I have a workaround posted here. WOTC just released a new executable that should work with these original directions. I updated the download link below. I haven't tested this yet, so let me know how it goes!
- Your macOS/OS X version must be 10.8 or greater
- Your Mac must support OpenGL 4.0 or greater (you can find out whether it does here)