After graduating, Onstad joined the Vancouver 86ers of the Canadian Soccer League in 1987. From 1988 to 1989, he played for the Winnipeg Fury, also of the CSL. He moved to a third CSL team, the Toronto Blizzard, for 1990 and 1991. Onstad returned to the Fury in 1992, winning both the CSL Championship and the CSL Goalkeeper of the Year Award. Onstad moved to the APSL in 1994, playing for the Toronto Rockets. After a season with the Rockets, he moved to the Montreal Impact for 1995. In 1996, he played indoor soccer for the Edmonton Drillers of the NPSL. He returned to outdoor soccer in 1997, playing for the Toronto Lynx, again of the A-League. In 1998, Onstad moved to his first American team, the Rochester Raging Rhinos, with whom he won the A-League title, and was also named the league's Goalkeeper of the Year after allowing only 13 goals in 26 regular season games. Onstad remained with Rochester for 1999, was named All-League second team, and helped the Rhinos become the only minor league team to win the U.S. Open Cup since the foundation of Major League Soccer. In 1999, Onstad was signed by Dundee United of Scotland, although he never saw any playing time with the first team. He returned to Rochester in 2001, and although a wrist injury kept him out of action for most of the year, he finished second in the league in goals against the following year. In 2003, Onstad was signed by fellow Canadian Frank Yallop to the San Jose Earthquakes as a replacement for Joe Cannon. Onstad filled in quite well, helping lead the Earthquakes to their second MLS Cup in three years, and winning the MLS Goalkeeper of the Year Award. He had another excellent year in 2005, supporting the Earthquakes as they went undefeated at home and winning the MLS Supporters' Shield, and winning the MLS Goalkeeper of the Year Award for a second time. He was named to the MLS Best XI in both 2003 and 2005. Along with the rest of his Earthquakes teammates, he moved to Houston for the 2006 season. Onstad played every minute of the regular season for the Dynamo. After the 2010 MLS season Houston declined Onstad's contract option and he elected to participate in the 2010 MLS Re-Entry Draft. Onstad became a free agent in Major League Soccer when he was not selected in the Re-Entry draft. Shortly thereafter, Onstad announced his retirement on December 21, 2010.
Coaching
Onstad joined D.C. United as an assistant coach alongside Chad Ashton. It was announced on February 17, 2011, that Onstad would come out of retirement to play for D.C. due to the injuries to Steve Cronin and Bill Hamid. His contract with D.C. United expired on May 31, 2011, and Onstad returned solely to his duties as an assistant coach. Onstad left D.C. in January 2013 and joined Toronto FC as Chief Scout and Manager of Football Partnerships. Shortly after the firings of general manager Kevin Payne, and director of team and player operations Earl Cochrane, Onstad left the organization.
