Cowell was born in Brighton, Sussex, on 7 October 1959, but soon afterwards his family moved to Elstree in Hertfordshire. His father, Eric Cowell, was a successful estate agent and quantity surveyor, while his mother, Julie Dalglish, was a glamorous socialite with a background in theatre.
Between them they already had four children from previous relationships, three boys – Tony, John and Michael – and a girl called June. They were keen to have a child together, but following complications their first son, Stephen, died a week after his birth. Julie then suffered further heartbreak with two miscarriages, Finally, she fell pregnant again, this time, though, everything was fine and she gave birth to a baby boy called Simon. Eric and Julie brought up Simon alongside his three half brothers, his half sister and his brother Nicholas, who was born two years after Simon.