Alan Louis Oppenheimer was born on April 23, 1930 in New York City, New York the son of Irene (née Rothschild; 1904-1991) and Louis E. Oppenheimer (1901-1984), who was a stockbroker.

Coincidentally, in his first appearance on Hogan's Heroes in 1967, he played a Nazi officer seeking information about the Manhattan Project, of which J. Robert Oppenheimer (no relation) was the Director of the Los Alamos Laboratory which built the bomb.

Oppenheimer is recognized as the voice of many characters, often for Filmation in the 1970s and 1980s, such as Mighty Mouse, Ming the Merciless on Flash Gordon, the Overlord on BlackStar, Skeletor, Man-At-Arms and Mer-Man from Filmation's 1980's cartoon He-Man and the Masters of the Universe, and the voice of Prime Evil in the 1986 TV series, Filmation's Ghostbusters. Other notable voice roles include Thundarr the Barbarian, Vanity on The Smurfs, Rhinokey and Crock from The Wuzzles and Falkor, Gmork, Rockbiter, and the Narrator from 1984's The NeverEnding Story. In the early 1990s, Oppenheimer was the voice of Merlin in The Legend of Prince Valiant. He also provided the voice of Barkerville in the Pound Puppies TV special. He also voiced Fraidy Cat on Fraidy Cat in 1975.

Alan showed himself well suited to the science fiction genre in the 1973 cult classic Westworld, where he played the head IT technician. He has also appeared in three Star Trek series, always playing a different character. He appeared in the Star Trek: The Next Generation episode "Rightful Heir" as a Klingon cleric, Koroth, a primary instigator of the cloning of Kahless; on Deep Space Nine as a Starfleet Captain Declan Keogh in command of the USS Odyssey; and as an alien ambassador in Voyager.

As a character actor, Oppenheimer has had diverse roles in popular American television programming, from playing a Nazi in Hogan's Heroes, to playing an Israeli secret agent as well as a double-agent KAOS scientist on Get Smart, to being the second actor to play Dr. Rudy Wells in The Six Million Dollar Man (Martin Balsam played the role in the pilot telemovie). Oppenheimer took over as Rudy starting with the second film, "Wine, Women and War" and kept playing up until the introduction of the bionic woman in 1975, whereupon Martin E. Brooks took over as Wells until cancellation). He was the original Mickey Malph (Ralph Malph's dad) on Happy Days. He played a recurring role during the first two seasons of St. Elsewhere as Helen Rosenthal's husband, Ira. He had a recurring role as Mayor Alvin B. Tutwiller on Mama's Family.

Oppenheimer was married to costume designer Marianna Elliott, with whom he had three children. The couple divorced but then remarried many years later in 1992, until her death in 2003. He was married to professional tennis player Marilyn Greenwood between marriages. That union ended in divorce.

Alan Oppenheimer appeared as film Director Cecil B. DeMille in the 1994 Broadway production of Andrew Lloyd Webber's Sunset Blvd.

Oppenheimer worked on The Transformers, most notably as two contrasting characters, the pacifist Beachcomber and the bellicose Warpath. His rendition of Seaspray was remarkably similar to Mer-Man, including the gurgling effects. He took over the voice of Roger Smith's butler Norman Burg in the second season of The Big O. He was the voice of the unseen Alistair Crane on the soap opera Passions up until 2004, when the character was made fully visible and played by David Bailey. More recently, he provided the voice of the Scientist for the 2009 film 9 and Batman's butler Alfred Pennyworth in Superman/Batman: Public Enemies.