Mullard was born in a humble background in Islington, London. He started work at the age of 14 as a butcher's assistant, and joined the Army at 18. It was there that he began boxing, becoming champion of his regiment. When he left the army after three years, he had a short stint at boxing professionally. This ended after 20 fights over three years, following a knock-out from which he lost his memory. He married Florence Rose in the second quarter of 1939. He is recorded in the 1939 Register as a general labourer living with his parents at 35 Douglas Street, Islington. He rejoined the army in the Second World War, becoming a sergeant major in the Royal Artillery.
Following the end of the war in 1945, Mullard sought work as a stuntman at Pinewood and Ealing film studios, from which he drifted into uncredited bit-parts in British films such as Oliver Twist (1948), The Lavender Hill Mob (1951) and The Ladykillers (1955).
Mullard also appeared in Ladies Who Do (1963), Morgan! (1966), The Great St Trinian's Train Robbery (1966), Chitty Chitty Bang Bang (1968) and Adventures of a Plumber's Mate (1978). In 1986, invited by Producer Victor Lewis-Smith, Mullard hosted an edition of Midweek on BBC Radio 4 to replace regular host Libby Purves during her temporary absence.
In 1967, Mullard recorded "I Love You, You Love Me"/"Was It Something I Said" on the Masquerade label (MA5001). This was followed the same year by an album Arthur Mullard of London (MQ 2003) The LP included Mullard's cover of the Beatles' "Yesterday", jokes and philosophy. More singles followed in the 1970s, including 1974's "Not Now Arthur"/"If I Only Had My Time Again" (BASFBA 1012), and in 1975 "I Only Have Eyes for You"/"One 'Fing 'N' Annuver" (RCA 2610) with "Yus My Dear"/"Arthur" (RCA Z639A) released in 1976.
Mullard (or "Arfur" as he was widely known) was regularly a guest in other programmes and television commercials. He and Watts also reprised their roles of Wally and Lily appearing in the film Holiday on the Buses (1973), the last feature-length version of the popular On the Buses comedy series of the time.
So popular was Mullard's character that a sequel, Yus, My Dear, was broadcast in 1976, in which Wally and his wife Lily (Queenie Watts) had moved out of their caravan into a council house. The series gained modest ratings, though it too received critical broadsides as one of Britain's worst-ever sitcoms.
He entered the UK Singles Chart in 1978 with his cover of "You're the One That I Want" (Pye 7N 46121) (from the film, Grease) with Hylda Baker, who was in her seventies. The single, which peaked at Number 22 in the UK, was taken from the album, Band On The Trot (Pye PKL 5576). The single was the last professional success of Mullard's life; there followed an uncredited narration on the Glenn Close-led live action 101 Dalmatians, released in 1996 after his death.
His wife Florence died of an overdose of sleeping tablets in 1961 after suffering from poor health. Her suicide was said by her daughter to be partly the result of the extreme physical and mental violence Mullard perpetrated against her. Mullard continued to live in a council house in Islington after his success, and spent much of his free time in local pubs. He died in his sleep on 11 December 1995, aged 85. Despite his history of child sexual abuse against her, his daughter was instrumental in nursing him in his dotage. He left her and one of her brothers £5,000 each; he left £250,000 to a children's home.
In 1996, the Sunday Mirror reported that a This Is Your Life special about Mullard was planned, until producers contacted his eldest son and discovered Mullard's history of extreme domestic violence, and years of sexual abuse against his daughter, starting when she was 13.