At Wheat Ridge High School from 1969–1972, Fosnes was "regarded by many as the best basketball player in state annals. Garnering three years all state, he led the Farmers to two seconds and a third in the state tournament. Averaging over twenty points per game for his prep career, he scored thirty-five points in the first three quarters of the 1971 state title game vs. Mitchell." Fosnes was named a Sunkist High School All-American in 1972 alongside Robert Parrish and Quinn Buckner.
Fosnes joined the Vanderbilt Commodores in 1972 as a part of a historic freshman class, "Roy Skinner's famed 'F-Troop', a nickname coined by Nashville Banner sports editor Dudley 'Waxo' Green."  In 1974, Vanderbilt would win the SEC Championship. Fosnes was an integral part of this team that included Jan van Breda Kolff, Butch Feher, and Joe Ford. Fosnes was selected to the NCAA All-Tournament Team in 1974, as the Commodores fell to Marquette and Notre Dame. During the same year, Fosnes became Vanderbilt's first Academic All-American, an accolade he would receive again the following year. No other Vanderbilt basketball player would attain the honor until Bruce Elder in 1993.
Fosnes was honored as a First Team All-SEC selection by the coaches in 1975 and 1976.
The 1975 Trojan Classic at USC selected Fosnes as the tournament MVP.
His career at Vanderbilt finished with a 15.5 PPG and .521 FG%.
Career, Single game, and Single season Records
His 1974-75 scoring average of 22.1 PPG ranks tied-4th for the highest single season scoring average in Vanderbilt history.
Fosnes currently ranks 3rd in career Made FGs with 651. His 1974-75 season of 238 Made FGs is the 4th highest total in Vanderbilt history.
Fosnes followed up his basketball career by pursuing a medical degree at the Vanderbilt University School of Medicine. He was "one of eighty-three students selected from 6100 applicants". After finishing his residency at Bayfront Medical Center with the University of South Florida in 1983, Fosnes moved to Springfield, Tennessee where he currently resides with his wife (Margot) and children (C.A. and Tyler).