He was one of the two favorite editors of director Joseph Losey, the other being Reginald Beck. However, after six collaborations with Losey, Mills gave an interview in which he was highly critical of Losey's frequent screenwriter, Harold Pinter, claiming that he had cut several of Pinter's typical pauses out of "The Servant" (1963) without anyone noticing. Pinter wrote him a furious note after reading this, and Losey never employed Mills again.