The former Celtic and Scotland defender was unveiled by United having left Thistle in second place in the Irn-Bru First Division, five points off leaders Morton but with two games in hand.
The new Tannadice manager said: "Obviously it was a big decision to make, to leave Thistle at this time.
"But it's one that feels right. It says what I think of Dundee United to do that. The things in place here are the things that attracted me to the job.
"It's an established SPL club with a fantastic playing squad, infrastructure and youth set-up."
McNamara has signed a three-year contract.