Jim Cronin was born in Yonkers, New York, to Italian-Irish parents. He was the son of a union official. He was educated at St Denis School and Lincoln High School. He had a number of jobs after leaving school before becoming a keeper at Bronx Zoo in the 1970s. While he was working there, he realised he wanted to work with animals. He moved to Kent in the UK to work in John Aspinall's zoo in 1980.