In 1963, Whitmore played Captain William Benteen in "The Twilight Zone" episode "On Thursday We Leave for Home." Whitmore also appeared in Route 66 on January 18, 1963, as Ralph Vincent in “A Gift for a Warrior.” He appeared twice in "Twelve O'Clock High". In 1965, Whitmore guest-starred as Col. Paul "Pappy" Hartley in Season 1, Episode 32 "The Hero" and as Col. Harry Connelly in 1966 Season 3, Episode 12 "The Ace". He also appeared in an episode of "Combat!" titled "The Cassock", as a German officer masquerading as a Catholic priest. In 1967, he guest-starred as a security guard in "The Invaders" episode, "Quantity: Unknown". That same year, Whitmore also appeared on an episode of ABC's "Custer" starring Wayne Maunder in the title role.