Peter Szymon Serafinowicz was born into a Catholic family in Liverpool's Gateacre suburb on 10 July 1972, the son of post office worker Catherine (née Geary) and scaffolder Szymon Serafinowicz Jr. His father was born and raised in Surrey to a Polish mother and a Polish–Belarusian father and later moved to Liverpool as an adult. Serafinowicz has a brother, James, a film producer, and a sister Helen, a writer who was married to Irish comedy writer Graham Linehan from 2004 to 2020.
At age three, Serafinowicz moved with his family to the Belle Vale district of Liverpool where he attended Our Lady of the Assumption Primary School. The family moved back to Gateacre when he was 14 and he attended St Francis Xavier's College in neighbouring Woolton. He later said: "I had a very happy childhood, but Belle Vale was very rough. I was only about three when we moved there, but I can still remember it looking very shiny, and it was all landscaped. But it was a very poor area, and it became scruffy quite quickly. [...] Gateacre is traditionally seen as one of the posh areas of Liverpool [but] it wasn't really that much posher!"
