Born in Silkeborg, Bak became a professional in 2002 for Team Fakta where he rode with fellow Dane Allan Johansen. In 2004 they both switched to BankGiroLoterij where Lars Bak would gain his first professional win, but as the BankGiroLoterij team stopped after the 2004 season, both Bak and Johansen went to Team CSC in 2005.
Here, Bak won the Danish Road Racing Championship, and in the later half of the season he showed his strength and talent as a good upcoming rider, as he won the respected ten stage Under 25 race, the Tour de l'Avenir. He won the leader's jersey by sprinting to the win in a group of four riders on the first stage, a position he defended through the following nine stages bar one, including a time trial and a number of hilly stages.
In 2011 he finally made his Tour de France debut where he worked hard for Mark Cavendish's sprint train and finished off by taking part in a breakaway on the last stage. Bak joined Lotto–Belisol for the 2012 season.