As a child, Waddell was often told stories in a lively manner. This inspired him and "the love of story" stuck with Waddell ever since. He aspired at a young age to be a football player and signed for Fulham FC youth team - Waddell reflects that he scored a hat-trick on his debut in adult football but wound up as a goalkeeper.

When it became clear to him that his future did not lie as a professional footballer, Waddell turned to his other love and began to write. Originally writing for adults, his first real success was a comic thriller "Otley", which was made into a film starring Tom Courtney and Romy Schneider. After moving back to Northern Ireland in the late sixties he wrote books that reflected on the changing situation in his native land. Soon his love of storytelling would pull him into the medium of children's literature.

In 1972 he went into a church to stop some vandals and got caught up in an explosion in Donaghadee, Co Down - an experience that took him some years to ovecome. As an author, nearly all of Waddell's stories are inspired by events and/or places in his life at the foot of the Mourne Mountains. As he humorously claimed, "I’ve been blown up, buried alive and had cancer as an adult, and survived all these experiences, so I’m a very lucky man."

In 2004, Waddell was awarded the Hans Christian Andersen Award for Children's Literature