During the 1960s, Rennie made guest appearances on such series as The Barbara Stanwyck Show, The Americans, Route 66 (a portrayal of a doomed pilot in the two-part episode "Fly Away Home"); Alfred Hitchcock Presents; Perry Mason (one of four actors in four consecutive episodes substituting for series star Raymond Burr, who was recovering from surgery); Wagon Train (a 90-minute colour episode as an English big game hunter); The Great Adventure (in an instalment of this anthology series about remarkable events in American history, he portrayed Confederate President Jefferson Davis); Daniel Boone, (in the episodes "The Sound of Wings" and "First in War, First in Peace"); Lost in Space (another two-part episode—as an all-powerful alien zookeeper, "The Keeper", he worked one last time with his Third Man co-star Jonathan Harris); The Time Tunnel (as Captain Smith of the Titanic, in the series' premiere episode); Batman (as the villainous Sandman, in league with Julie Newmar's Catwoman); three episodes of The Invaders (as a benign variation of the Klaatu persona, culminating in a parallel plot also involving an assembly of world leaders); an episode of I Spy ("Lana"); and two episodes of The F.B.I.; and was a THRUSH agent in an episode of The Man From U.N.C.L.E. (1967 TV series) ("The Thrush Roulette Affair"/Barnaby Partridge). Also Branded.