Stempniak grew up in West Seneca, New York, where his family lived near an ice rink. He mixed his desire to play hockey with his parents' urgings to go to college by attending Dartmouth, and after scoring 21 goals as a sophomore, he was selected in the fifth round (No. 148) in the 2003 NHL Draft.
The Blues signed Stempniak after his senior season, and he quickly earned a call-up to St. Louis in 2005-06, finishing with 27 points (14 goals, 13 assists) in 57 games. He put up NHL career-highs of 27 goals and 52 points the following season, playing on a line with David Backes and Jay McClement.