Tjärnqvist was drafted by the Dallas Stars with the 96th overall, third round pick in the 1999 NHL Entry Draft. During the 2004–05 NHL lockout, Tjärnqvist played with HV 71 in the Elitserien along with fellow NHL'ers Manny Malhotra, Jonathan Cheechoo, and Bryan McCabe. On February 12, 2007, he was traded to the Phoenix Coyotes, along with a first round pick in the 2007 NHL Entry Draft, for Ladislav Nagy. In 2017 Tjärnqvist officially announced his retirement.