How to start launcher Minecraft in offline mode in a mac

Open your launcher while the Wi-Fi is off. If it shows "Play Now", it means authentication succeeded and you didn't cut your Wi-Fi in time.

You can fix this by closing and reopening while the computer is disconnected from the internet.

I wanted to say that, if you want to play in a server with online mode set to false, your launcher does not need to show the button "play offline"

You can be logged in and connected to the internet and still be able to play on the server. What I am basically saying is you can join offline servers with a premium account, so you do not need turn off your Internet and play offline. I hope this solves your problem and helps you.