In 1935, he married Dorothy Yockel. They remained married until her death in 1978 and had one daughter.
Allyn Joslyn was born in Milford, Pennsylvania, the son of a mining Engineer. On stage from age 17, Joslyn scored as a leading man in such Broadway productions as Boy Meets Girl (1936) and Arsenic and Old Lace (1941), appearing in the latter as beleaguered theatrical critic Mortimer Brewster. However, Hollywood didn't see Joslyn as a leading type. Thus, he spent most of his film career playing obnoxious reporters, weaklings, and formless "other men" who never got the girl, while stars such as James Cagney and Cary Grant took the roles he originated on Broadway.
Among his more notable film appearances were tough flier Les Peters in Only Angels Have Wings (1939); George in The Great McGinty (1940); the caustic Director in No Time for Comedy (1940); reporter Chic Clark in My Sister Eileen (1942); Albert Van Cleve, Don Ameche's cousin and snobbish rival for the affections of Gene Tierney in Heaven Can Wait (1943); an eccentric poet in The Shocking Miss Pilgrim ; Dangerous Blondes (1943); the wise sheriff in Moonrise (1948); and the cardshark disguised as a woman in Titanic (1953). In the sprightly B picture It Shouldn't Happen to a Dog (1946), Joslyn was for once cast in the lead, even winning heroine Carole Landis at the fade-out.
A prolific radio and television performer, Joslyn was a co-star of the 1953–1954 ABC sitcom Where's Raymond?, in which he played Jonathan Wallace, brother of the title character Raymond Wallace, the role of Ray Bolger. Betty Lynn played June Wallace, Joslyn's young wife in the series. Joslyn and Lynn left the series in the 1954–1955 series, when it was renamed The Ray Bolger Show.
In 1957, Joslyn co-starred in eight episodes as literary agent George Howell in the short-lived CBS sitcom The Eve Arden Show. He also guest-starred in the NBC western series The Californians. In 1959, he was cast as Quag in the episode "Gold Sled" of the ABC/Warner Brothers western series The Alaskans, starring Roger Moore. In the 1960–1961 season, Joslyn guest-starred on Pat O'Brien's ABC sitcom Harrigan and Son. He played one-half of the title role on the 1962 TV sitcom McKeever And The Colonel. That same year, he guest-starred in ABC's crime drama series Target: The Corruptors with Stephen McNally and Robert Harland. He was featured as well in some episodes of ABC's The Addams Family playing the role of beleaguered Sam L. Hilliard, whose efforts as a school administrator and Politician were continually undone by the Addams family.
On January 21, 1981, Allyn Joslyn died of heart failure at the Motion Picture & Television Country House and Hospital in Woodland Hills in Los Angeles, California, at the age of seventy-nine. He is interred at Forest Lawn Memorial Park in the Hollywood Hills of Los Angeles.