In February 1924, Lethbridge married Sylvia Robertson in a ceremony held at Salisbury Cathedral. Together they moved into a house known as The Lodge in Waterbeach. There, their first two sons were born: Christopher John in March 1925 and Hugh Periam in July 1926. In 1927, they moved to Mount Blow, a house in Shelford designed by architect Edwin Lutyens. It was there that Sylvia gave birth to a daughter, Belinda Mary, in April 1930. Sylvia suffered from mental illness however, resulting in repeated hospitalisation. Lethbridge meanwhile devoted much of his time to yachting around the British Isles, sometimes taking family members with him. Over the course of the 1930s he self-published a series of books featuring his own sketches and engravings of maritime scenes. He also deepened his interest in the paranormal during this period, coming to believe that an acquaintance of his was a genuine psychic and observing an unidentified flying object in Bracknell.