Dan Thomassen started his volleyball career in Orange County, playing under Dan Glenn at Newport Harbor, and under Mike Delasandro and John Wallce for Balboa Bay Volleyball Club. He continued playing at UCSD as a middle blocker for four years. His first coaching experience came in 1994 when he assisted the Balboa Bay Boys 16's to a third place finish at the Junior Olympics, in Austin. Coach Thomassen started coaching girls' club volleyball for Sunwest (now Epic) in San Diego, 1999, and in 2003 coached the boys at The Bishop's School in La Jolla.
Dan came to Sage Hill in 2003 as a chemistry and physics teacher and assisted both the boys' and girls' volleyball teams, both of which had their best seasons since the opening of the school. Dan continued as the head girls' volleyball coach in 2004 where they finished in the semifinals of CIF-SS, Div. 4A. In 2005, the Sage Hill girls achieved the school's first ever CIF championship with a straight set victory over rival St. Margaret's.

Dan still competes in beach volleyball and enjoys surfing, tennis, golf, and traveling with his friends or any of his nine brothers and sisters.