Selected by the Guelph Storm in the 15th round (293rd overall) in the 2001 OHL Priority Selection, Callahan was chosen as the Storm’s co-rookie in 2002-03 after registering 14 goals and 17 assists in 59 games. In 2003-04, he appeared in 68 games with the Storm and collected 68 points (36 goals and 32 assists, and in the postseason, tied for second in team scoring with 21 points (13 goals and 8 assists) in 22 games.
After being drafted by the Rangers, he returned to Guelph in the fall of 2004, and was named team captain. His offensive production fell off somewhat during the season and when he came to the Rangers training camp that fall, Callahan was offered an AHL contract. Rather than accepting the contract, Callahan Decided instead to return for an overage year with the Guelph Storm. Callahan put up very impressive numbers with the OHL team during the 2005-06 season. In 62 regular season games, the gritty winger recorded 84 points and 126 PIM. He was signed to an NHL contract on May 11, 2006.
Callahan underwent knee surgery after the 2005-06 season and was still finding his way when training camp opened in the fall of 2006. However, after being assigned to Hartford, the gritty winger quickly showed that he could produce offensively on a consistent basis at the professional level. He was called up to the NHL 3 times during the 2006-07 regular season, the last time on March 15, 2007. Since that time, Callahan has remained with the NHL team. After a slow start to the 2007-08 season, Callahan suffered a knee injury, which kept him out of the lineup for more than a month.