Mair left the Buddies yesterday after agreeing to mutually terminate his Paisley contract and has now become Archibald's second signing of the transfer window, following loan star Lyle Taylor in making the move to Firhill.
Archibald was thwarted in a bid to capture the former Dundee, Dundee United and Aberdeen defender in the summer, but after Mair failed to hold down a regular place in the Saints side, the veteran has decided to bring an end to his five years at St Mirren Park.
He will provide competition for Jags defensive trio Conrad Balatoni, Gabriel Piccolo and Aaron Muirhead and could make his bow in the clash with Kilmarnock today.
Archibald said: "Lee is a player that we tried to sign in the summer, but we couldn't agree a deal so I'm delighted that we have managed to do so now.
"He'll help add some experience at the back.
"It's not so much a case of those there not playing well, it's just that they lack a bit of experience and Lee will be able to help them out in that respect.
"He's a good player and is fit and raring to go and will be in the squad for today."
After his Paisley exit was confirmed, Mair thanked the Buddies faithful for their backing on his Twitter page yesterday afternoon.
He said: "It has been a great five years. The time is right to move on though.
"Good luck and thanks to all the Buddies for the support over the years."