The relegated Paisley side, who will play in the forthcoming SPFL Championship after their nine-year Premiership stay ended last season, have given him a one-year deal.
Carswell played 98 times in all for Motherwell after first breaking through in 2011 when he made his debut against Hearts.
He fell out of Ian Baraclough’s starting line-up in the second half of last term, however, and went almost four months without a first-team start.
Now Ian Murray has moved for him after he took charge of the club last month and Carswell is his second new recruit after Scott Agrew.
The 21-year-old told St Mirren’s official website: “I’m looking forward to a new challenge and getting the club back up where it belongs.
“I came out to the training ground last week and was very impressed with Ian and the facilities there. I can’t wait to get started.”