Sellers was born on 8 September 1925, in Southsea, a suburb of Portsmouth. His parents were Yorkshire-born William "Bill" Sellers (1900–62) and Agnes Doreen "Peg" (née Marks, 1892–1967). Both were variety entertainers; Peg was in the Ray Sisters troupe. Although christened Richard Henry, his parents called him Peter, after his elder stillborn brother. Sellers remained an only child.
Sellers was two weeks old when he was carried on stage by Dick Henderson, the headline act at the Kings Theatre in Southsea: the crowd sang "For He's a Jolly Good Fellow", which caused the infant to cry. The family constantly toured, causing much upheaval and unhappiness in the young Sellers' life.
Sellers maintained a very close relationship with his mother, which his friend Spike Milligan later considered unhealthy for a grown man. Sellers's agent, Dennis Selinger, recalled his first meeting with Peg and Peter Sellers, noting that "Sellers was an immensely shy young man, inclined to be dominated by his mother, but without resentment or objection." As an only child though, he spent much time alone.
In 1935 the Sellers family moved to North London and settled in Muswell Hill.