O'Malley died of cardiovascular disease in San Juan Capistrano, California, shortly before his 81st birthday. He was survived by his wife Fay (1926–2002) and their two children.

At the end of 1935 Hylton and O'Malley came to the United States to record with a band composed of American Musicians, thus emulating Ray Noble and Al Bowlly. The venture was short-lived. O'Malley remained in the US, known professionally as J. Pat O'Malley (to avoid confusion with another film actor named Pat O'Malley); he had a long and varied acting career, including the 1943 film Lassie Come Home as "Hynes".

O'Malley guest-starred in 1951 as a sheriff on Bill Williams's syndicated western series, The Adventures of Kit Carson. From 1950-55, he appeared in five episodes of The Philco-Goodyear Television Playhouse. From 1951-57 he was cast in eight episodes of another anthology series, Robert Montgomery Presents. Other television work from this period include roles in Walt Disney's Spin and Marty film (1955) and serial (1955-57) as the always-faithful ranch steward, Perkins.

A DVD version of the Adventures of Spin & Marty from the 1955 season was released in 2005 as part of the Walt Disney Treasures series.

In 1956 he guest-starred in one of the last episodes, "The Guilty", of the NBC legal drama Justice, based on case files of the Legal Aid Society of New York. In 1958 he was a guest star in "Peter Gunn" (Season 1, Episode 3, "The Viscious Dog") as Homer Tweed.

O'Malley was cast as Walter Morgan in the 1959 episode "The First Gold Brick" of the NBC western series The Californians. In 1959-1960 he made eight appearances as Judge Caleb Marsh in Black Saddle. In 1959 he was cast as Dr Hardy in an early episode of Hennesey, starring Jackie Cooper. In season 3, Episode 10, titled "The Medicine Man", of the television series Wanted: Dead or Alive starring Steve McQueen, O'Malley played the character of Doc. He also appeared in the role of a bank President in an episode of The Real McCoys titled "The Bank Loan", which was released 15 January 1959.

In 1960 O'Malley made guest appearances on The Tab Hunter Show, The Law and Mr. Jones, Johnny Midnight, Johnny Staccato, Harrigan and Son, Adventures in Paradise, The Islanders, Going My Way, and The Tall Man. He made numerous guest appearances on CBS's Perry Mason, including as the defendant in the 1960 episode "The Case of the Prudent Prosecutor" and as the murderer in the 1961 episode "The Case of the Roving River".

In 1961 O'Malley appeared in 3 episodes of Tales of Wells Fargo, in different roles. In the episode "The Has-Been" he had the title role, playing a fading entertainer grieving over the loss of his wife. In one poignant scene, O'Malley displayed his song and dance talent as he performed for an imaginary audience in an abandoned dance hall. Later that year he guest-starred in the television version of Bus Stop and the following year appeared in two episodes of The Twilight Zone, "The Fugitive" and "Mr. Garrity and the Graves". He also guest-starred twice on The Lloyd Bridges Show in that series' 1962-1963 season. He then co-starred with Spring Byington in the 1964 episode "This Train Don't Stop Till It Gets There" of The Greatest Show on Earth.

During the 1963-1964 season O'Malley appeared in eight episodes of My Favorite Martian. In 1964-1965 season, he was cast in Wendy and Me. O'Malley appeared in the Hogan's Heroes episode "How to Cook a German Goose by Radar" in 1966, and the 1967 episode "D-Day at Stalag 13". In 1966 he also appeared as Ed Breck in the episode "Win Place and Die" of Jack Sheldon's short-lived sitcom Run, Buddy, Run. He appeared occasionally as "Vince" in The Rounders. In the 1966 episode "The Four Dollar Law Suit" of the syndicated western series Death Valley Days, O'Malley played the Lawyer for Alfred Hall (Strother Martin), a country chicken farmer who sues an insurance company for underpaying him four dollars after his chicken coop burns to the ground.

Walt Disney engaged O'Malley to provide voices for animated films such as the Cockney coster in the "Supercalifragilisticexpialidocious" sequence in Mary Poppins (1964); Cyril Proudbottom, Winkie, and a policeman in The Adventures of Ichabod and Mr. Toad (1949); and the role of Colonel Hathi and the vulture Buzzie in The Jungle Book (1967). His voice can be heard in Alice in Wonderland (1951), in which he performs all the character voices in "The Walrus and the Carpenter" segment (besides Alice), including Tweedledum and Tweedledee, the Walrus, the Carpenter, and Mother Oyster. O'Malley also provided the voice of Br'er Fox in Song of the South (1946) when James Baskett was unavailable. Actor Dick Van Dyke has said that O'Malley was his dialect coach on Mary Poppins, attributing his infamous Cockney accent in that film to O'Malley.

In 1969 O'Malley portrayed Carol Brady's (Florence Henderson) father in the first episode of ABC's The Brady Bunch. The name "Fleming" was used in O'Malley's first two appearances on The Fugitive (Season 1, See Hollywood And Die; Season 3, Crack In A Crystal Ball). In 1973 O'Malley starred with Shirley Booth in the short-lived comedy A Touch of Grace. He made several appearances on Maude between 1973 and 1975; and he performed on other series such as It Takes a Thief, One Day At A Time, Emergency!, Adam-12, The Practice, Three's Company, and Taxi. O'Malley also appeared on the ABC television series Family in 1979 and on Barney Miller in 1979 and 1981.

Born in Burnley, Lancashire to Irish parents, O’Malley began his career in entertainment in 1925 as a recording Artist and then as principal singer with Jack Hylton and his orchestra in the United Kingdom from 1930 to 1933. Known at that time as Pat O'Malley, he recorded more than four hundred popular songs of the day. He began a solo recording career in 1935 in parallel with his work with Hylton.