Born in Washington, DC, raised in Canada and trained for the stage in England, he spent several years honing his craft in various UK stage productions, including "The Glass Menagerie", "A Comedy of Errors" and "Deathtrap". Frewer made his stage debut in "Bent", a drama about the treatment of homosexuals by the Nazis during the Holocaust and made his London stage debut in Israel Horowitz's drama "The Indian Wants the Bronx".
Frewer's TV debut was a bit role (American at the Bar) in the BBC miniseries adaptation of F Scott Fitzgerald's "Tender Is the Night" (1983; shown in the US on Showtime in 1985). His American debut was the 1984 NBC miniseries "The First Olympics--Athens 1896". Fame came with his casting as "Max Headroom" (1984) in a British TV-movie and subsequent series. Playing the dual role of journalist Edison Carter and the computer-generated Max, Frewer was able to play both dramatic scenes and to develop his smart-alecky, irreverent comic style. While attempts to duplicate the success of "Max Headroom" met with limited success in the US (a cable talk show on Cinemax and a short-lived series on ABC, both 1987), American TV viewers came to recognize the computer-generated figure primarily through a series of commercials for Coca-Cola. Frewer moved on to the series "Doctor, Doctor" (CBS, 1989-91) where his manic, sarcastic comic style flowered. Originally produced as a short-run 1989 summer series, the critical reaction prompted the network to return it to its schedule as a replacement in the fall of 1989. Although never a ratings success, "Doctor, Doctor" developed a cult audience and managed to eke out a two-year run. As Providence, RI physician Mike Stratford, Frewer made use of improvisation and exhibited a madcap wit, drawing comparisons with Robin Williams from many critics. A subsequent series, "Shaky Ground" (Fox, 1992-93), in which he played a sarcastic aerospace worker, failed to capture viewers. Other TV appearances have allowed Frewer to demonstrate his dramatic abilities. He has made guest appearances on various series ("St. Elsewhere", "Miami Vice") and was effective as Japanese historian Edwin O Rieschauer in the American Playhouse presentation "Long Shadow" (PBS, 1994), as an arsonist allied with the evil Flagg (Jamey Sheridan) in the 1994 ABC miniseries "The Stand", based on the Stephen King novel and as Alexander Haig in "Kissinger and Nixon" (TNT, 1995).
Frewer's film work has been of uneven quality. He made his feature acting debut as a cadet in "The Lords of Discipline" (1983) and followed with small roles in big screen disappointments like "Supergirl" (1984) and "Ishtar" (1987). He and Drew Barrymore (as his daughter) were trapped in a trailer park by a psychopath in the thriller "Far From Home" (1989). That same year, Frewer had one of his best screen roles to date as Big Russ Thompson, Rick Moranis' neighbor in Joe Johnston's "Honey, I Shrunk the Kids". He was one of the many characters who come into contact with "Twenty Bucks" (1993) and was noted for his comic energy in "National Lampoon's Senior Trip" and his nefarious computer genius in "Lawnmower Man 2: Beyond Cyberspace" (both 1995).