In 1963-1964, Lockwood starred as a young U.S. Marine second lieutenant named william T. ("Bill") Rice in the NBC series The Lieutenant. This drama, about the peacetime Marines, was produced by the creators of Star Trek (Gene Roddenberry) and The Man from U.N.C.L.E. (Norman Felton). The series co-starred Robert Vaughn as Lieutenant Rice's immediate superior, Captain Raymond Rambridge. Despite moderately good reviews, The Lieutenant's Saturday night time slot, opposite CBS' Jackie Gleason's American Scene Magazine, caused its cancellation after 29 episodes. In 1964, Lockwood guest-starred as Major Gus Denver in the first season of 12 O'Clock High, in episode 9, "Appointment at Liege", and again in 1965 in episode 29, "V For Vendetta". He also guest-starred as Lt. Josh McGraw in season 2, episode 4, "The Idolator" of 12 O'Clock High.