He started in the film industry at Carlton Hill Studios in London at the age of fourteen, and went on to work extensively for Disney as a camera assistant throughout the late 1950s and early 1960s before getting his break as camera operator on The Saint television series (1966-68). In 1969, he was camera operator on his first James Bond film, On Her Majesty's Secret Service, and went on to operate on another four films in the series (The Spy Who Loved Me, Moonraker, For Your Eyes Only and Octopussy) before becoming director of photography on The Living Daylights and License to Kill.
Mills was the principal director of photography on some twenty-one films before retiring from active film-making in 2001, having also directed two films (Bloodmoon and Dead Sleep) in Australia. Following his retirement he became a tutor at the National Film & Television School in Beaconsfield, England, before writing his autobiography, Shooting 007 and other Celluloid Adventures, which was published by The History Press on 1st July 2014.