Mills edited seven films with Joseph Losey, an American director who settled in Britain after being blacklisted at home. Mills edited Losey's first British film, "The Sleeping Tiger" (1954). Perhaps the most successful film of their collaboration was "The Servant" (1963), which is considered to be a high point in Losey's career. Mills had a fairly public artistic dispute with screenwriter Harold Pinter about the editing of "The Servant" that likely led to the end of Mills' collaboration with Losey. Losey and Mills did make one final film together ("King & Country" – 1964) that did not involve Pinter. Reginald Beck, who had edited two films with Losey prior to 1964, subsequently became Losey's principal editor for the rest of his career.