McNamara, who has signed a three-year contract, leapt to the top of United's wishlist after the SPL side parted company with Peter Houston on Monday.
The 39-year-old will be assisted by former Celtic and Scotland team-mate Simon Donnelly, who held a similar position at Firhill.
United chairman Stephen Thompson said: "It was important we moved quickly to appoint a new manager, but it was equally important we undertook extensive homework to ensure the correct appointment.
"We have worked on this for some time and after receiving great interest in the position and undertaking a rigorous interview process, it was clear to the board that Jackie was the outstanding candidate to take the club forward in the manner we wish.
"It is with great delight and excitement for the future that I announce his appointment."
McNamara said: "I am thrilled and proud to be named manager of Dundee United and believe I have been handed a fantastic opportunity.
"This is an established Scottish Premier League club with a fantastic playing squad, infrastructure and youth set up and I cannot wait to start working with the players.
"This club has very strong foundations with the majority of the first team squad under contract beyond this summer and a very strong youth structure in place.
"I am sure the Dundee United supporters will appreciate how I like to play the game and I look forward to working with everyone connected with the club to take us forward."
During his career McNamara scored 10 goals in 255 appearances for Celtic and won 33 caps before taking over as Thistle boss in April 2011 following the dismissal of Ian McColl.