What officer rank is Jack Arnold seen wearing in photographs?

Jack Arnold is seen in photographs wearing the uniform of a First Lieutenant. The rank is stated directly in descriptions of his character, so “First Lieutenant” is the single canonical answer.

The photographs refer to Jack’s earlier military service, before viewers see him primarily as Kevin Arnold’s demanding father and as a middle manager at NORCOM. His Korean War veteran status gives the character a backstory rooted in the military experiences of the postwar generation. The uniform details reinforce that history without making Jack an active-duty character during the main events of the series.

The rank can be confused with the ranks offered as alternatives. Sergeant is an enlisted rank, while Major and Captain are higher commissioned officer ranks. The show’s character information specifically identifies the photographed uniform as that of a First Lieutenant, not merely an unspecified officer’s uniform.