Harkness played college basketball for the Loyola Ramblers, where he was captain of the 1962–63 team that won the 1963 NCAA national championship.

Before playing in the professional leagues, the 6-foot-3-inch (1.91 m) Harkness was a star at DeWitt Clinton High School and Loyola University Chicago.

Harkness was enshrined in history as he shook hands with Joe Dan Gold, the white captain of Mississippi State, prior to tip-off.

[8] Though his professional career was relatively short, he left his mark in the record books on November 13, 1967, when he hit an 88-foot-long (27 m) game-winning buzzer beater to lead the Pacers past the Dallas Chaparrals, 119–118.

He worked with the Southern Christian Leadership Conference in the early 1970s,[14] and he served as executive director of the Indianapolis chapter of 100 Black Men, a national organization dedicated to supporting and training young African American males.

[16][17] On July 11, 2013, in the Oval Office of the White House, Harkness and former Loyola teammates John Egan, Les Hunter and Ron Miller met with President Barack Obama to commemorate the 50th anniversary of the school's 1963 national championship.

[18] In September 2013, Harkness and the entire 1963 Loyola Ramblers NCAA Championship basketball team was inducted into the Chicagoland Sports Hall of Fame.