Dennis was a standout player at Dominguez High School, Harbor College, and Pepperdine University (HOF selection in 1981) before moving into a long NBA career. He was also a coach for the Boston Celtics and Los Angeles Clippers. He led Harbor to the 1975 California State JC title, averaging 18 points and 12 rebounds per game. In his one-year at Pepperdine he averaged 16 points, 6 rebounds and 3 assists per game, while leading the Waves to the WCC title and to the NCAA West Regional semi-finals. Then he became a second round NBA draft pick.

He led the Seattle Sonics to its only NBA title in 1979 and was selected as Finals MVP. He won two more NBA titles with the Boston Celtics as a 6-4 starting guard. The Celtics retired his number 3 jersey. He was a five-time All-Star and 9-time All-Defensive player. He was elected to the Naismith Hall of Fame in 2010. His brother Joey was a talented player at Banning and ASU. Dennis passed away at age 52 in February of 2007.