Alex was born in Hertfordshire and raised in Windsor. His mother, Lee, is an interior designer. His father, Richard Pettyfer, is a fellow actor. His parents divorced and Alex now has younger half-brother, James, from his mother's re-marriage to a property developer, Michael. James is a junior tennis player.
At age 7, Alex started modeling for Gap's children range. He also appeared in TV commercials for other brands. When he was older, he modeled for Burberry to fund his traveling adventures.
Alex attended various private schools. During this time, he enjoyed performing in school plays. At age 13, he auditioned and won a role in the TV movie Tom Brown's Schooldays (2005) (TV). At age 15, he went to Los Angeles and was cast as Alex Rider, the main character in _Alex Rider: Operation Stormbreaker (2006)_. This was his breakthrough role and he was nominated for a Young Artist Award and an Empire Award for this role.