The 19-year-old has made 16 starts for Well this season and was previously under contract until the end of the current campaign.
Cummins, who moved to Fir Park after being released by Everton, told MFC TV: "I am really happy to get things confirmed.
"I feel as if I've had a fairly good season so far but it's only half done. I've got to carry on the way I am, listening to everyone who is trying to help me and hopefully improve further.
"I certainly didn't want to be chasing my tail, worrying about my future but now that I have things sorted I can concentrate on developing.
"That's not to say it's time to put the tools away, I will keep playing and training as if I am battling for a new deal and hopefully keep my place in the starting XI."
- Sports & Recreation