His father was a medical doctor in General Practice. McGlashan joined the Royal Flying Corps in 1916, later the RAF. After constantly flying over German lines, he was Mentioned in Despatches, awarded the MC and the Croix de Guerre with Palm, the citation reading, He has accomplished all his missions with the greatest devotion, and on many occasions has succeeded in reconnaissance at very low altitude in perilous conditions under violent enemy fire. In 1934 he married Hilda Cameron-Smith (died 1975), and in 1979 he married Sasha Baldi.