Terry began his career working in a photographic unit for an airline at London's Heathrow Airport. During this time, he photographed a sleeping figure in a waiting area who, by happenstance, was revealed to be Britain's Home Secretary. O'Neill thereafter found further employment on Fleet Street with The Daily Sketch in 1959. His first professional job was photographing Laurence Olivier. His reputation grew during the 1960s. In addition to photographing the decade's show-business elite such as Judy Garland, the Beatles and the Rolling Stones, he also photographed members of the British Royal Family and prominent politicians, showing a more natural and human side to these subjects than had usually been portrayed before. He was awarded The Royal Photographic Society's Centenary medal 'in recognition of a sustained, significant contribution to the art of photography' in 2011. O'Neill's work was featured in the 2015 Proud Chelsea exhibition 'Sinatra at 100: A Century in the Making'.