Humphries appeared in numerous West End stage productions including the musicals "Oliver!" and "Maggie May", by Lionel Bart, and in stage and radio productions by his friend Spike Milligan. At one time he was invited to play the leading role of Captain Martin Bules in "The Bedsitting Room" which had already opened successfully at the Mermaid Theatre and was transferring to the West End. Humphries performed with Milligan in the 1968 production of "Treasure Island" in the role of Long John Silver. He described working with Milligan as "one of the strangest and most exhilarating experiences of my career".