The former Celtic captain has moved to Tannadice with assistant Simon Donnelly after United agreed a deal with Partick Thistle.
McNamara replaces Peter Houston, who left the club on Monday.
Jackie McNamara factfile
1973: Born October 24
1991: Begins career with Dunfermline, making 64 appearances.
1995: Moves to Celtic after Parkhead boss Tommy Burns agrees £650,000 deal with Pars.
1996: Named Scottish Professional Footballers Association young player of the year.
1997: Wins first of 33 Scotland caps in friendly with Wales at Rugby Park.
1998: Lifts his first trophy with Celtic as Glasgow giants win the Scottish Premier Division, ending Rangers' bid for 10 titles in succession. Also handed Scottish PFA Players' player of the year award and named in Craig Brown's Scotland World Cup squad which travels to France.
2001: Martin O'Neill takes over as Hoops boss but McNamara often finds himself on the bench.
2004: Fights his way back into contention and plays almost every SPL game of the 2004/2005 season.
2005: Handed captain's armband when Paul Lambert is ruled out with injury. However, quits club later in the year when Celtic fail to offer him a new contact and signs Bosman deal with Wolves. His first season at Molineux is wrecked by a cruciate knee ligament injury.
2007: Moves to Aberdeen but quits just 10 months into his two-year deal citing frustrations with "travelling and injuries".
2008: Signs for Falkirk but 18 months later is then loaned to Partick Thistle, where he breaks his leg in a game against Ayr Utd, ending his 2010 season.
2011: Takes over as Jags caretaker boss in April before landing full-time deal a month later.
2012: His first full season sees him lead Partick to sixth place in the Scottish First Division.
2013: January 30 - Confirmed as new boss of Dundee United, replacing departing manager Peter Houston. He leaves Thistle sitting second in the First Division table, five points behind leaders Morton but with two games in hand.