The 40-year-old former Scotland international said the decision to commit his future to the Terrors was one of the easiest he has made since taking up the Tannadice reins in January 2013.
McNamara said: “I am enjoying the job more than ever. My relationship with the board of directors and the chairman is the strongest I have experienced in football, and working with the quality players available to me at Dundee United makes the job enjoyable.
“The supporters are excellent and I continue to have a great relationship with them. Signing this extension is one of the easiest decisions I have made as manager.
“I am proud to be Dundee United’s manager and I see great potential at the club.
“We have made a lot of progress in the first year here but can achieve a lot more, and my desire to take Dundee United to a higher level is my motivation each day.”
McNamara guided the team to a top-six finish in the Scottish Premier League and a Scottish Cup semi-final in his first season, and chairman Stephen Thompson was thrilled to have secured his services for a prolonged period.
“The board is excited by the job that Jackie has been doing and his desire to build strong foundations is in line with how we see the club progressing,” he said.
“His commitment to developing young players and giving them the chance to prove themselves on the park is creating positivity for the future and, importantly, entertaining the fans.
“Some of the football this season has been sensational and United supporters have shown hunger for his philosophy by buying half-season tickets in record numbers and creating a positive atmosphere at Tannadice on match days.”