He was born in Ogden, Utah. Foulger attended the University of Utah, and started acting through his participation in community theatre. He made his Broadway debut in March 1920 in a production of Medea featuring Moroni Olsen, and performed in four more productions with Olsen on the 'Great White Way', back-to-back, ending in April 1922. He then toured with Olsen's stock company, and ended up at the Pasadena Playhouse, where he both acted and directed.
Foulger made his first three films in 1932 and 1936 with small roles in Night World (1932), The Little Minister, and The President's Mystery, the latter based on a story by Franklin Delano Roosevelt. However, his film career did not start in earnest until 1937 after he performed opposite Mae West in a racy 'Adam and Eve' sketch on the Edgar Bergen-Charlie McCarthy network radio program which resulted in West being banned from the airwaves almost immediately. (Foulger played the voice of the serpent.) From this point on, Foulger worked steadily in motion pictures.
He played many parts: storekeepers, hotel desk clerks, morticians, professors, bank tellers, ministers, confidence men, and a host of other characterizations, usually timid, whining, weak-willed, shifty, sanctimonious or sycophantic. His earliest films show him clean-shaven, but in the 1940s he adopted a wispy mustache that emphasized his characters' worried manner. When the mustache went gray in the 1950s, he reverted to a clean-shaven look. Foulger was a resourceful actor and often embellished his scripted lines with memorable bits of business: in The Falcon Strikes Back, for example, hotel clerk Foulger announces a homicide by bellowing across the lobby: "Mur-der! Mur-der!'
In real life, Foulger was not as much of a pushover as the characters he played. In one memorable incident at a party he threatened to punch Errol Flynn for flirting with his wife, the actress Dorothy Adams, with whom he was married from 1921 until his death in 1970.
In the 1940s, Foulger was part of Preston Sturges' unofficial "stock company" of character actors, appearing in five films written by Sturges, The Great McGinty, Sullivan's Travels, The Palm Beach Story, The Miracle of Morgan's Creek (recreating the role of McGinty's secretary he played in The Great McGinty) and The Great Moment. In "A" pictures, such as those of Sturges', Foulger would often not receive a screen credit: in B movies such as 1939's The Man They Could Not Hang, he would get more substantial billed parts.
By the late 1950s, Foulger was so well established as a mild-mannered worrywart that just the showing of his face would receive a welcoming audience laugh (this happens in the cameo-laden Frank Capra comedy Pocketful of Miracles). In a humorous coup, the actor was cast against type for the most prominent role of his career: he played the Devil opposite The Bowery Boys in Up in Smoke, and was billed in ads and posters as one of the film's three stars.
Beginning in 1950, Foulger made over 90 appearances on television, in programs such as Death Valley Days, I Love Lucy, The Cisco Kid, My Little Margie, The Man Behind the Badge, The Lone Ranger, Maverick, Lawman, The Red Skelton Show, Rawhide, Wagon Train, Bonanza, Burke's Law, Daniel Boone, Hazel, The Patty Duke Show, The Monkees, Perry Mason, Laredo and Gunsmoke. He played multiple-episode characters on Dennis the Menace ("Mr. Timberlake"), Lassie ("Dan Porter") and The Andy Griffith Show ("Fred, the hotel clerk"). On Petticoat Junction he played two recurring roles: "Mr. Guerney" and engineer "Wendell Gibbs".
Notable later television credits included the 1959 Twilight Zone episode "Walking Distance" – in which Gig Young tells Foulger, who is playing a drugstore counterman, that he thinks he's seen him before, to which Foulger replies: "I've got that kind of face" – the short-lived comedies My Mother the Car (as one of the villain's browbeaten advisors) and Captain Nice (as the hero's often silent father), and The Mod Squad, his last appearance in episodic television.
Byron Foulger's last film appearances were in The Love War, a made-for-TV movie, and There Was a Crooked Man..., both in 1970. He died of heart problems in Hollywood on 4 April of that year at the age of 70, and is buried in Inglewood Park Cemetery in Inglewood, California. Foulger died on the same day the final episode of Petticoat Junction was broadcast, in which he played train engineer Wendell Gibbs in the 1968–1969 season. He was survived by his wife Dorothy Adams and daughter, Rachel Ames, both actresses.