The 25-year-old joins the Buddies following the expiry of his contract with A-League side Newcastle Jets.
Born in Melbourne, Millar joined Melbourne City in 2015 and also had spells with South Melbourne, and Central Coast Mariners.
He spent a spell on loan with English League One side Shrewsbury Town last season and joins Saints until the end of the 2021-22 campaign, subject to international clearance.
He told stmirren.com: "I know Scotland pretty well because my grandparents were born in Glasgow and my parents lived in Glasgow.
"I was really excited when St Mirren came in for me and I just want to hit the ground running.
"I've got a lot of pace and I like to get up and down the wing. I'm also a very competitive person so you'll definitely see that.
"Hopefully I have a good week of training and can be in a position to have some impact this weekend."
Saints manager Jim Goodwin said: "We're delighted to get Matt in.
"I had spoken to his agent around six or seven weeks ago but we thought he was going to go to an English team. That didn't happen and the fact he was still available made it a really easy one.
"He'll bring a bit of pace and power to the team. He's a good type that's played at a decent level and I've no doubt that he'll fit in well with the group."