Fraser-Simson's biggest success was the score for the operetta The Maid of the Mountains, which played at Daly's Theatre in London in 1917 and finally closed after 1,352 performances. This was, at the time, a phenomenal run second only to that of Chu Chin Chow. Several songs from this work (not all of them by Fraser-Simson) have remained "standards" ever since. Fraser-Simson's best-known songs for this show included "Love will Find a Way", "Farewell" and "Husbands and Wives". The Maid of the Mountains has been frequently revived by both professional and amateur groups and was filmed in 1932. It was one of the three most important musical hits of the London stage during World War I (the other two being a revue, The Bing Boys Are Here and the musical Chu Chin Chow. Music or scenes from all of these have been included as background in many films set in this period, and they remain intensely evocative of the "Great War" years. Audiences wanted light and uplifting entertainment during the war, and these shows delivered it.
After The Maid of the Mountains, Fraser-Simson wrote music for more operettas and musicals, including A Southern Maid (premiered in Manchester in 1917 and produced at Daly's in London after Maid closed in 1920); Our Peg (1919, with a libretto by Harry Graham and Edward Knoblock at Prince's Theatre); Missy Jo (1921 touring); Head over Heels (Adelphi Theatre, 1923); Our Nell (1924, Lyric Theatre – a rewrite of Our Peg for the star Nell Gwynne), The Street Singer (1924, 360 performances at the Lyric, starring Phyllis Dare); and Betty in Mayfair (1925, Adelphi Theatre).
Fraser's music tended towards the old-fashioned European romantic songs, in contrast to the ragtime, jazz and other American dance music that began to be used in musicals during World War I. His other stage works include a ballet, Venetian Wedding (1926) and incidental music for The Nightingale and the Rose (1927).
In later years, Fraser-Simson lived the life of a country squire at Dalcross Castle, a home that he bought in Scotland. He married Cicely Devenish. He was an avid sportsman, enjoying golf, tennis, shooting and fishing.
Fraser-Simson died at a nursing home in Inverness, Scotland, following a fall on a stone staircase at his home in nearby Croy, Highland, at the age of 71.