Gardiner worked in almost 100 movies. He started film work in crowd scenes, making his big film break in 1926 in the silent film The Lodger, by Alfred Hitchcock.

Gardiner started as a super on stage and eventually became well known on the West End stage. "He appeared in British revues, plays, and films before delighting Broadway audiences in 1935 with a wallpaper imitation act in At Home Abroad." His other Broadway credits include Little Glass Clock and An Evening with Beatrice Lillie.

His Hollywood film debut came in 1936. During his career, he was cast in numerous roles, often as a British butler. One of his most famous roles was that of Schultz in Charlie Chaplin's The Great Dictator. He also performed memorable turns as Beverly Carlton (a parody of Noel Coward) in The Man Who Came to Dinner, the spurned "almost-husband" in The Doctor Takes a Wife and Christmas in Connecticut.

He also recorded a curious and eccentric classic called "Trains" which was regularly played on a 1950s British radio programme called Children's Favourites. This record, consisted of a tipsy-sounding Gardiner reciting a monologue, which he first introduced in the 1935 Broadway revue At Home Abroad, about steam railway engines (which he claimed were 'livid beasts') and impersonating both the engines themselves and the sound of trains running on the track. This latter he famously characterised as 'diddly-dee, diddly-dum' to mimic the sound pattern as the four pairs of bogie wheels ran over joins between the lengths of track. (A sound no longer heard since welded rail joins were introduced.) "Trains" was released as a 78 and 45 by English Decca Records (F 5278) which remained on catalogue into the 1970s. At the end of the record, Gardiner signs off with "Well folks, that's all: back to the asylum." He was summoned to Buckingham Palace to give a performance in person.

On 4 October 1956, Gardiner appeared with Greer Garson as the first two guest stars in the series premiere of NBC's The Ford Show, Starring Tennessee Ernie Ford. In 1956, he was the guest star on "The Millionaire" on the episode "The Story Of Waldo Francis Turner". He made other guest appearances on television sitcoms of the 1960s, including Fess Parker's ABC series, Mr. Smith Goes to Washington and Stanley Holloway's Our Man Higgins. He also appeared in the 1964 Perry Mason episode, "The Case of the Ugly Duckling," as Business owner Albert Charity, and in an episode of Alfred Hitchcock Presents ("Banquo's Chair"). His last major role was alongside Phyllis Diller in her 1966-1967 ABC series, The Pruitts of Southampton. Also in 1967 he made a guest appearance on Petticoat Junction, in the episode "Uncle Joe and the Master Plan", as Gaylord Martindale.

Gardiner died of a heart attack at his home in Westwood, California, on 7 July 1980. He was survived by his wife.