He played professionally in the National Football League (NFL) with the Chicago Cardinals in 1930 and the New York Giants in 1931.
[3] He was a flexible player on both sides of the ball, recording three rushing touchdowns and also showing a threat using the forward pass and picked up an interception while playing defense.
[5] In 1931, Flenniken moved to the New York Giants,[6] where he was a part of the "committee of quarterbacks" along with Red Smith and Hap Moran.
[7] For the Giants, Flenniken saw less playing time but in his four games and one start[8] he recorded successes in both passing and rushing.
[9] Flenniken was the 14th head football coach at Geneva College in Beaver Falls, Pennsylvania and he held that position for two seasons, from 1928 until 1929.