When his boss at the GPO, Alberto Cavalcanti, moved to Ealing Studios, Hamer was invited to join him there. He gained some experience as a director by substituting for colleagues and contributed the 'haunted mirror' sequence to Dead of Night (1945). He followed this with the three Ealing films under his own name for which he is best remembered: Pink String and Sealing Wax (1946), It Always Rains on Sunday (1947), both featuring Googie Withers, and the black comedy Kind Hearts and Coronets (1949), with Dennis Price and Alec Guinness. He was an alcoholic, who by the time of his last film as director, School for Scoundrels (1960) was "often battling terrifying DT hallucinations" (i.e. alcohol withdrawal symptoms, occurring only in patients with a history of alcoholism). BFI Screenonline writes that Hamer was "a recovering alcoholic" and that "he fell off the wagon during production [of School For Scoundrels], was sacked on the spot ... and would never work in the industry again."
Hamer, who was homosexual in an era when it was illegal in the UK, died of pneumonia at the age of 52 at St Thomas's Hospital in London. He was buried at Llandegley and there was a large crowd at his funeral.
Hamer's career "now looks like the most serious miscarriage of talent in the postwar British cinema", according to film critic David Thomson.