Koskie was drafted by the Minnesota Twins in the 26th round of the 1994 Major League Baseball draft, and subsequently made his MLB debut on September 9, 1998. He established himself in 1999 as the Twins' third baseman of the future by hitting.310 and knocking in 58 runs. In 2000, he batted.300 with a.400 on-base percentage and in 2001 he had his most productive offensive season when he hit 26 home runs, had 103 RBI, and scored 100 runs. Koskie signed a three-year, $17-million contract with the Toronto Blue Jays on December 14, 2004. An injury-plagued season cut his productivity and playing time, as he struggled with a.249 average, with 11 home runs, 36 RBI, and 4 stolen bases in 97 games. Despite his frequent injuries, Koskie was regarded as one of the more athletic third basemen in the game. On January 6, 2006, he was traded by the Blue Jays to the Milwaukee Brewers for pitcher Brian Wolfe. The deal was widely viewed as a salary dump for the Blue Jays after the team traded for Troy Glaus. With several players on the roster capable of playing third base and no assurances of regular playing time from Blue Jays general manager J.P. Ricciardi, Koskie was traded just a year after being the Blue Jays' 2004 offseason marquee free agent signing. While the Brewers acquired a veteran infielder with a solid glove and bat to anchor their very young infield, Koskie's later seasons were marred by injuries, including missing most of the 2006 season with post-concussion syndrome from an injury he received on July 5, 2006. "If I can't play, I at least want my life back", he said. "I'm hoping to do something to help the team this year. I want to play baseball again. If I can play baseball, I know I'm fine." The injury caused him to miss the entire 2007 season. In, Koskie worked out at the Minnesota Twins spring training camp and played for Team Canada in the World Baseball Classic. Koskie signed a minor league deal with an invitation to spring training with the Chicago Cubs on February 28. On March 21, 2009, Koskie retired. While confident in his abilities, he did not want to risk his health. His fear of injury led to a diagnosis of post traumatic stress disorder.
Post-retirement
Koskie was a Planet Fitnessfranchisee in the Minneapolis-Saint Paul area from 2010 to 2016. He recounted his experiences in a January 29, 2016 article for The Players' Tribune. As of 2016, Koskie lived in Plymouth, Minnesota with his wife and their four sons.