Coffin was born in the gold and silver mining community of Mammoth in Juab County in central Utah and reared in the state capital of Salt Lake City. He began acting while he was in high school and thereafter joined traveling stock companies. He earned a Bachelor of Arts degree in speech from the University of Washington at Seattle, Washington. He worked as a news analyst and sportscaster until spotted by a Hollywood talent scout. His stolid looks were said to have served him well in his later roles.
In 1940, Coffin appeared as Phillips, along with Milburn Stone, later of Gunsmoke, and I. Stanford Jolley, in Chasing Trouble, a comedy espionage film. He is perhaps best known for his role as Jeff King in Republic Pictures' King of the Rocket Men, the first of three serials starring the "Rocketman" character, who would later be paid homage to through the Dave Stevens-created comic book character The Rocketeer, which was adapted into a Walt Disney film in 1992. During the 1940s and into the early 1950s Coffin appeared in many other movie serials, including Mysterious Dr. Satan, Sky Raiders, Holt of the Secret Service, Perils of Nyoka, Federal Agents vs. the Underworld, Radar Patrol vs. Spy King, and Captain Video: Master of the Stratosphere.
In 1955, he joined Peter Graves, William Schallert, and Tyler McVey in the episode "The Man Who Tore Down the Wall" of NBC's Hallmark Hall of Fame. He had guest starred in the series Adventures of Superman, sometimes playing a "good guy", sometimes a "bad guy".
He also had a role in the very first TV episode of The Lone Ranger, as Captain Reid of the Texas Rangers, the older brother of the man who would become The Lone Ranger after his brother and four other comrades were murdered by outlaws. From 1951 to 1955, he appeared eight times as Colonel Culver in the Bill Williams syndicated television series, The Adventures of Kit Carson. He appeared nine times as banker Tom Barton in the syndicated half-hour color western series, The Cisco Kid, starring Duncan Renaldo and Leo Carrillo. In 1956, Coffin appeared in different roles in six episodes of the syndicated series, Judge Roy Bean, with Edgar Buchanan, Jack Buetel, and Jackie Loughery. He also appeared as a guest star in the ABC western series, The Life and Legend of Wyatt Earp, starring Hugh O'Brian in the title role.
Coffin appeared in the role of Col. Willis Murdock on the ABC/Warner Brothers western series, Colt .45 in the 1960 episode, "The Cause". On February 9, 1960, Coffin appeared as Grey Gordon in "The 10 percent Blues" of the ABC/WB crime drama, Bourbon Street Beat, with Andrew Duggan, Richard Long, and Van Williams. He guest starred as well on the ABC/WB western series, The Alaskans.
Coffin played the lead as Captain Thomas H. Rynning in his own syndicated series 26 Men, based on official files of the Arizona Rangers in the final days of taming the "Old West" before Arizona statehood in 1912. Kelo Henderson appeared with Coffin in the role of Deputy Clint Travis.
In 1966, Coffin played Joshua Bean, the older brother of Roy Bean (played by Tom Skerritt) in the episode "A Sense of Justice", of the syndicated western series Death Valley Days. The segment is set in San Diego, California, where Joshua Bean was the founding mayor. He appeared in other Death Valley Days episodes too, including "The Hat That Huldah Wore" with Anna-Lisa and "Hugh Glass Meets The Bear" with John Alderson, both also in 1966.
In 1954, Coffin committed a noted blooper on the Climax! live television anthology series, in "The Long Goodbye", in which Coffin's character was depicted as lying dead. The actor did not realize he was still on frame, resurrected himself, and walked off camera. Despite this mishap, the actor was cast and appeared in another episode of Climax!, "Escape From Fear", in 1955.
He appeared in comedy too, including episodes of Father Knows Best, Hey, Jeannie!, I Love Lucy, Batman, and Walter Brennan's The Real McCoys.
Coffin died of lung cancer at the age of eighty in Santa Monica, California. His ashes were scattered at sea.The Corpse Vanishes (1942)
The Lady Pays Off (1951)
Northwest Territory (1951)
Fireman Save My Child (1954)
Creature with the Atom Brain (1955)
The Night the World Exploded (1957)
The Crawling Hand (1963)
Iron Angel (1964)
Good Neighbor Sam (1964)
Zebra in the Kitchen (1965)
The Barefoot Executive (1971)
Night Call Nurses (1972)