Sheppard graduated from the Royal Academy of Dramatic Art 1956-58 before spending 12 years as Associate Artist with the Royal Shakespeare Company. He appeared on Broadway in 1966 with Marat/Sade and later in 1975 with Sherlock Holmes. He won the Los Angeles Drama Critics Circle Award for The Homecoming in 1995, at the Matrix Theatre. He is the father of actor Mark Sheppard.
He appeared in the 1978 television film The Nativity.
He starred alongside John Terry in the 1980 fantasy film Hawk the Slayer.
He appeared in the 1988 comedy horror film Elvira, Mistress of the Dark as Elvira's Great-Uncle Vincent Talbot who was an evil warlock.
He appeared in the 1992 comedy film There Goes the Neighbourhood.
He has appeared in two roles on the science fiction series Babylon 5 (1994–1998), first as a "Soul Hunter" in the episode of the same name and secondly as Narn war leader G'Sten, an uncle of main character G'Kar, in "The Long Twilight Struggle". He was also a runner-up for the role of Ambassador G'Kar on the series, though the role eventually went to Andreas Katsulas. He also played a computer hologram for an A.I. programme during the first and second season of seaQuest DSV.
Sheppard was cast in the 1996 adventure game Zork Nemesis, playing the live-action role of Bishop Francois Malveaux, one of the four alchemists central to the plot of the game.
In 2000, he voiced the character Ignatius Cheese in the game Escape from Monkey Island.
In 2002, he voiced the wizard Merlin in the Justice League episode "A Knight of Shadows".
Sheppard played the role of Captain Archibald Witwicky in the 2007 live action film Transformers.
In December 2009, Sheppard voiced the part of Santa in Disney's Christmas special Prep & Landing. Following that, he returned as the voice of Santa in the sequels, Prep & Landing: Operation: Secret Santa and Prep & Landing: Naughty vs. Nice.
On 4 December 2010, Sheppard appeared on Hallmark Channel Original Films Farewell Mr. Kringle as Kris Kringle.
Sheppard appeared in a seventh-season episode of It's Always Sunny in Philadelphia titled "How Mac Got Fat". He played a priest forced to Listen to Mac's confession – which involved accepting no blame of his own while demanding that the priest smite his friends, claiming that they made him fat.
