Knap was born and raised in Milwaukee, Wisconsin, and graduated from Riverside High School, where he was an All-City selection in football in 1934.
Along with three other Milwaukee freshman, he accepted a football scholarship to the University of Idaho and played on the Vandals freshman team in the fall of 1935,then lettered for three seasons (1936–38) on the varsity under head coach Ted Bank. Among his UI teammates were future head coaches and administrators Lyle Smith and Steve Belko. Other teammates included future Idaho athletic director Leon Green, and NFL pros George "Iron Man" Thiessen (Rams), Stonko Pavkov (Steelers), Dean Green (Eagles), Richard "Truck" Trzuskowski (Lions), and Hal Roise (Bears).
As a senior in 1938, Knap was a second-team All-Coast selection at end, the only Vandal to make any of the three teams.
Knap was also a pitcher and utility player for three seasons on the varsity baseball team, and a member of the Sigma Alpha Epsilon fraternity.