Arthur was the son of a Sussex solicitor; he was educated at boarding school in Uppingham in Rutland.
Treacher was a veteran of World War I serving as an officer in the Royal Garrison Artillery; his father had served with the Sussex Volunteer Artillery before his son's birth. After the war, he established a stage career and in 1926, he went to America as part of a musical-comedy revue called Great Temptations. He was featured in the 1930 Billy Rose production Sweet and Low.
He began his film career in the 1930s, which included roles in four Shirley Temple films: Curly Top (1935), Stowaway (1936), Heidi(1937) and The Little Princess (1939). Scenes intentionally put the 6' 4" Treacher standing or dancing side-by-side with the tiny child actress. Treacher filled the role of the ideal butler, and he portrayed P.G. Wodehouse's perfect valet character Jeeves in the films Thank You, Jeeves! (1936) and Step Lively, Jeeves (1937). He also played a valet or butler in several other films, includingPersonal Maid's Secret, Mister Cinderella and Bordertown.
In 1961 and 1962, he and William Gaxton starred in Guy Lombardo's production of the musical Paradise Island, which played at theJones Beach Marine Theater.
In 1962, he replaced Robert Coote as King Pellinore (with over-the-title star billing) in the original Broadway production of Lerner and Loewe's musical Camelot, and he remained with the show through the Chicago engagement and post-Broadway tour that closed in August 1964.
In 1964, Treacher played the role of stuffy English butler Arthur Pinkney in two episodes of The Beverly Hillbillies. Pinkney mistakenly believed the hillbillies were the domestic servants of the family he was hired by, while the hillbillies believed Pinkney was a boarder at their Beverly Hills mansion.
Treacher played the role of Constable Jones in Disney's Mary Poppins and made many guest appearances on U.S. television, in addition to being Merv Griffin's announcer and sidekick on The Merv Griffin Show from 1965–70 ("...and now, here's the dear boy himself, Meeeer-vin!") When Griffin switched from syndication to CBS in 1969, the network brass insisted that Treacher was too old for the show, but Griffin fought to keep Treacher and eventually won out. However, when Griffin moved his show to Los Angeles the following year, Treacher stayed behind, telling Griffin "at my age, I don't want to move, especially to someplace that shakes!"
During this period of latter-day popularity, Treacher also capitalised on his name recognition through the use of his name and image for such franchised business concerns as the Call Arthur Treacher Service System (a household help agency) and Arthur Treacher's Fish and Chips. The restaurant chain became popular in the 1970s and grew to nearly 900 outlets, although it is unclear whether or not Treacher had any financial involvement with the company. The fish and chips chain continues to exist, although there are believed to be only around 45 franchises left throughout the United States.
He was survived by his wife, Virginia Taylor who married him in 1940. Treacher's ashes were scattered into the Atlantic Ocean.