Which military branch did Jack Arnold serve in?

Jack Arnold served in the US Marine Corps. The character is described as a Korean War veteran who joined the Marines after graduating from college, making the Marine Corps the canonical answer rather than the Army, Navy, or Air Force.

Jack’s military background helps explain his stern, disciplined personality in The Wonder Years. He belongs to a generation shaped by the Great Depression and wartime service, while his son Kevin grows up amid the social changes of the late 1960s and early 1970s. The contrast between Jack’s outlook and the younger generation is a recurring part of the series.

A common mix-up comes from the show’s broader military setting. Jack works for NORCOM, a military defense contractor, and several characters have different service connections: Brian Cooper serves in the Army and is killed in Vietnam, while Jack’s son Wayne later considers joining the Army. Those details do not change Jack’s own branch.