Mack Flenniken was born January 24, 1905, in Ruston, LA, and attended North Side High School in Fort Worth, TX.

Following high school, Mack played one year of college football at Centenary College in Shreveport, LA, under Beaver County Sports Hall of Fame coach Bo McMillin. Mack then traveled north to follow Coach McMillin to play football at Geneva College in Beaver Falls.

In the mid-1920s, 6'-1" Mack was an offensive and defensive football standout for Geneva during his time with the Golden Tornadoes. A backfield ace, Mack played end, tackle, and fullback with extraordinary spirit and strength. In 1926, with no quarterbacks available, Mack was assigned to call signals. Excelling at the position, Mack was placed on the All-Conference team. In 1926 and 1927, Mack lettered in football at Geneva, while also starring on the basketball and track and field teams. In 1926, Mack played with fellow Beaver County Sports Hall of Famers Al Maglisceau and Ernie Meyer on the Geneva football team, along with their legendary quarterback Cal Hubbard, in the team's 16-7 historic upset of a powerful Harvard team.

Because of his football prowess, in 1928, Mack became the 14th head coach at Geneva. He would hold the coaching position until 1929, with a record at Geneva of 7-11-1.

Mack's professional NFL career started in 1930, when he got his start with the Chicago Cardinals (nka the Arizona Cardinals), under coach and player and Pro Football Hall of Famer Ernie Nevers. A flexible player at home at Chicago's Comiskey Park and on the road, Mack played offense and defense and was a threat throwing the ball using the forward pass. That year, Mack also recorded three rushing touchdowns. In 1931, Mack moved to the New York Giants under Pro Football Hall of Fame Coach Steve Owen. Although he saw less playing time in his four games and one start, Mack recorded successes in rushing and passing.

Mack was the head coach of the Arlington Heights High School football team from 1937 to 1942, and was the assistant football coach for Idaho in the Pacific Coast Conference from 1951 through 1953. Mack served in the U.S. Army Air Force from 1940-1945, being discharged at the rank of major. Mack was inducted into the Geneva College Athletics Hall of Fame as part of its 2020 Legacy Hall of Fame Class.

George "Mack" Flenniken, a U.S. Army veteran, died May 26, 1956, at age 51, in Geneva, PA. He is buried in Sun Valley, ID.