[1][2] Aged seven, he played Puck in a production of William Shakespeare's A Midsummer Night's Dream at Manhattan's Public School 46, and decided to become an actor.

Sloane took a Wall Street job as a stockbroker's runner, but when his salary was cut in half after the stock market crash of 1929, he began to supplement his income with radio work.

He became the sleuth's assistant on WOR's Impossible Detective Mysteries,[3] played the title character's sidekick, Denny, in Bulldog Drummond[5] and went on to perform in thousands of radio programs.

[3][7][8] Sloane made his Broadway debut in 1935, playing Rosetti the agent in George Abbott's hit comedy, Boy Meets Girl.

At that time Julian had to content himself with being an indistinguishable voice in crowd scenes, envying this "hallowed circle" that included Sloane, Kenny Delmar, Arlene Francis, Gary Merrill, Agnes Moorehead, Jeanette Nolan, Paul Stewart, Orson Welles, Richard Widmark,[12]: 9  Art Carney, Ray Collins, Pedro de Cordoba, Ted de Corsia, Juano Hernandez, Nancy Kelly, John McIntire, Jack Smart, and Dwight Weist.

Sloane recorded one program with The Mercury Theatre on the Air and became a regular player when the show was picked up by a sponsor and became The Campbell Playhouse.

Sloane moved with the rest of the company to Los Angeles to continue recording the show after Welles signed his contract with RKO Pictures.

He appeared on the NBC anthology series The Joseph Cotten Show, also known as On Trial, in the 1956 episode "Law Is for the Lovers", with co-star Inger Stevens.

On March 7, 1959, he guest-starred in an episode of NBC's Cimarron City titled "The Ratman", appearing alongside the show's star, John Smith.

[15] Later that same year, Sloane appeared as a guest in "Stage Stop", the premiere episode of John Smith's second NBC Western series, Laramie.