Leno was born in St Pancras, London, and began to entertain as a child. In 1864, he joined his parents on stage in their music hall act, and he made his first solo appearance, aged nine, at the Britannia Music Hall in Coventry. As a youth, he was famous for his clog dancing, and in his teen years, he became the star of his family's act. He adopted the stage name Dan Leno and, in 1884, made his first performance under that name in London. As a solo artist, he became increasingly popular during the late 1880s and 1890s, when he was one of the highest-paid comedians in the world. He developed a music hall act of talking about life's mundane subjects, mixed with comic songs and surreal observations, and created a host of mostly working-class characters to illustrate his stories. In 1901, still at the peak of his career, he performed his "Huntsman" sketch for Edward VII at Sandringham. The monarch was so impressed that Leno became publicly known as "the king's jester".
Leno also appeared in burlesque and, every year from 1888 to 1904, in the Drury Lane Theatre's Christmas pantomime spectacles. He was generous and active in charitable causes, especially to benefit performers in need. Leno continued to appear in musical comedies and his own music hall routines until 1902, although he suffered increasingly from alcoholism. This, together with his long association with dame and low comedy roles, prevented him from being taken seriously as a dramatic actor, and he was turned down for Shakespearean roles. Leno began to behave in an erratic and furious manner by 1902, and he suffered a mental breakdown in early 1903. He was committed to a mental asylum, but was discharged later that year. After one more show, his health declined, and he died aged 43.
Leno was born in St Pancras, London. He was the youngest of six children, including two elder brothers, John and Henry, and an elder sister, Frances. Two other siblings died in infancy. His parents, John Galvin (1826–1864) and his wife Louisa (née Dutton; 1831–1891), performed together in a music hall double act called "The Singing and Acting Duettists". Known professionally as Mr. and Mrs. Johnny Wild, they did not achieve much success, and the family struggled in poverty.
Having had very little schooling, and being raised by performers, Leno learned to entertain as a child. In 1862, Leno's parents and elder brothers appeared at the Surrey Music Hall in Sheffield, then performed in northern cities later in the year. In 1864, at the age of four, Leno joined his parents on stage for the first time, at the Cosmotheca Music Hall in Paddington, under the billing "Little George, the Infant Wonder, Contortionist, and Posturer".
When Leno was four years old, his alcoholic father died, aged 37. The family moved to Liverpool a few months later, where his mother married William Grant (1837–1896), on 7 March 1866. Grant was a comedian of Lancastrian and Irish descent, who performed in music halls throughout the British provinces under the stage name of William Leno. He was a seasoned actor and had previously been employed by Charles Kean in his theatre company at the Princess's Theatre in London. In 1866, the family home in Marylebone was demolished to make way for St Pancras railway station and, as a result, Leno's sister Frances was sent to live with an uncle, while his brother John, who performed occasionally with his parents, took full-time employment. Leno, his mother, stepfather and brother Henry moved north and settled in Liverpool, where they performed in various halls and theatres, including the Star Music Hall, but they often returned to London to perform in the capital's music halls.