Having left the RAF as a flight lieutenant on 21 August 1919, Slessor applied to rejoin and was offered a short-service commission at the same rank on 24 February 1920. In May 1921, he became a flight commander with No. 20 Squadron, which operated Bristol Fighters on the North-West Frontier of India. He joined the staff at the Directorate of Training and Staff Duties in the Air Ministry in February 1923. The same year, he married Hermione Grace Guinness; they had a son and a daughter. He attended the RAF Staff College, Andover, in 1924, and was promoted squadron leader on 1 January 1925.