The native of Lachine, Quebec, threw and batted left-handed and was listed as 6 feet 2 inches (1.88 m) tall and 182 pounds (83 kg) (13 stone).

[1] Harkness made his Major League Baseball debut on September 12, 1961 against the Phillies, working out a walk in five pitches against pitcher Chris Short as a pinch hitter in the ninth inning in a 19–10 loss at the Los Angeles Memorial Coliseum.

[1] He hit the first home run of his career on April 17 in the top of the second inning against Mike McCormick to drive in Daryl Spencer, as part of an 8–7 win over the San Francisco Giants at Candlestick Park.

[1] On July 28, after Harkness went one-for-four as the Mets' first baseman in a 9–0 loss to the Dodgers, he was traded to the Cincinnati Reds for infielder Bobby Klaus and assigned to Triple-A San Diego.

In 259 big-league games played through all or parts of four seasons, Harkness collected 132 hits, with 18 doubles and four triples accompanying his 14 home runs.