[3] His brother, Larry Cullen, was a retired captain in the United States Marine Corps, and helped inspire the voice of Optimus Prime.

[4] In 1968, he and Joan Stuart appeared as "Giles" and "Penelope" in L'Anglaise, a recurring segment about a French-Canadian man with an English-Canadian wife, on the CBC Radio comedy series, Funny You Should Say That.

[5] He honed his voice skills by working as a radio announcer, notably in his home town of Montreal on (then) MOR station CKGM doing the overnight and weekend swing shifts.

In 1974, Cullen was the announcer and a series regular (with Ted Zeigler and Billy Van) on The Hudson Brothers Razzle Dazzle Show.

"[4][7] Cullen later learned from his agent, Steve Tisherman, that he not only won the part of Prime, but also, to his surprise, the role of Ironhide as well, which he saw as a "home run".

"[8] He has also stated that he had no idea of Prime's popularity until the character's controversial death in the 1986 animated film, because the studio had never given him fan letters from children addressed to Optimus.

He played Mantor/Mantys in Coleco's 5 episode mini-series Sectaurs in 1986; Red Skull in the 1981 cartoon series of Spider-Man, Klaar and Zanzoar in Megas XLR and Mantus in The Pirates of Dark Water.

Cullen spent some of his early professional years in Montreal as a radio announcer/DJ on CKGM; and as a character in a then-popular local television kids show.

"[12] Additionally, he did voicework for the unreleased Blizzard Entertainment game Warcraft Adventures: Lord of the Clans,[13] and provided narration for the Star Wars fan film series I.M.P.S.

[14] Cullen is a voice announcer on Police Story 4: First Strike in advertisements for New Line Cinema, lending his experience as an ex-cop for the Santa Monica division to the cast.

He is a good friend of his longtime Transformers co-star and fellow voice actor Frank Welker, in contrast to the rivalry of their respective characters, Optimus Prime and Megatron.

At BotCon 2010, Hasbro named Cullen as one of the first four human inductees in the Transformers Hall of Fame for his role as the voice of Optimus Prime.