The 39-year-old, who has left Partick Thistle to take the job, has revealed that he is looking forward to working with an "established Scottish Premier League club".
"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," he told the club's official website.
"When I spoke to the board of directors, we shared the same vision and belief with regards to how Dundee United should progress. 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."
McNamara's first game in charge will be against Rangers in the Scottish Cup on Saturday afternoon.