After 27 goals in 40 games for the University of Maryland, the then 20 -year-old joined the German Bundesliga TSV 1860 München. In his two years in Germany Twellman graduated no match for the first team of the club but only played for the amateurs. After this disappointing time for him, he moved in 2002 to the Major League Soccer to New England Revolution. Even in his first season he was one of the most prolific players in the league and scored 23 goals for the Revs and was even with 52 points scorer in the league. In the same year he took second place as the best player in the league behind Carlos Ruiz, who was playing at Los Angeles Galaxy.
In the 2003 and 2004 season Twellman was plagued by several injuries and he was not back to his old form. 2005 but he ran on top form and was top scorer with 17 goals the MLS. In addition, he was named the best player of the season.
At the end of the 2010 season Twellman finished his career after he was no longer employed at New England Revolution.
On November 17, 2002 he played his first international match for U.S. national team against El Salvador.