Oakland traded him to Kansas City in 2004, along with Mike Wood, as part of a three-way trade that sent Carlos Beltran to the Houston Astros, Octavio Dotel from the Astros to the Athletics, and John Buck from the Astros to the Royals. Although he had been one of the most highly regarded prospects in the Oakland organization, he became available when the Athletics were able to sign Eric Chavez to a long-term contract.
Royals management did not expect Teahen to make the opening-day roster in ; however, after hitting .344 in spring training and after injuries sidelined potential third basemen Chris Truby and Chris Clapinski, the Royals announced Teahen would be their regular third baseman for 2005, and he made his major-league debut on April 4, 2005.
His first hit was a triple and he hit 4 homers in his last 17 games of 2005. Teahen hit .240 in his rookie season with KC. He had the lowest fielding percentage of all third basemen in the league, .947, and the lowest zone rating of all major league third basemen (.706).
In 2006, he hit .290 with 18 home runs and 69 RBIs. During July, he raised his performance level with 10 home runs and 35 RBIs during the 38 games ending August 2. He continued to hit well, but suffered an injury and had to have season-ending surgery on September 8.
Teahen recently obtained dual Canadian-American citizenship since his father originates from St. Marys, Ontario.
During the 2008 season, he hit TWO inside-the-park home runs http://fantasysports.usatoday.com/content/player_news.asp?sport=mlb&id=1155&line=247659
During important Teahen at-bats in the later innings of Royals' home games, a brief video montage is played on the Jumbotron. The screen is divided into four individual squares each showing different Teahen footage with the flashing digital 24 in the center while playing the 24 beep theme as background music. Teahen is 4-11 with 6 RBIs after the 24 montage has been played.