In 1980-81, big Dan Brennan enrolled at the University of North Dakota as an 18-year-old after finishing high school in B.C. During that freshman year with North Dakota, Brennan played in 37 games, scoring three goals and 12 points. He was primarily used as a defensive forward, assigned to check the other team's top scoring players. Brennan's all-round game improved each year and by his senior season he was a solid two-way player. He scored 28 goals and 65 points in 45 games and was named to the WCHA's First All-Star Team following the 1983-84 season.
Brennan caught the attention of the NHL's Los Angeles Kings, thanks to his strong skating and size, and he was inserted into their lineup for two games that same year. In 1984-85, Brennan signed a deal with the AHL's New Haven Nighthawks. He played in all 80 games, scoring 25 goals and 58 points. The following year he returned to the Nighthawks for 62 games and also got the call from the L.A. Kings on six occasions.
In eight NHL contests, Brennan tallied one point.