He studied at Cambridge University from 1931. While rusticated from Cambridge in 1932 due to a youthful indiscretion - he had bitten a policeman's nose - he met the society beauty Pamela Fletcher during a stag hunt on Exmoor. Their mutual lack of money delayed their marriage. In the meantime, he joined the Rifle Brigade in 1933 after he graduated. He served in Malta for three years. His father was created Baron Gowrie in 1935.
Hore-Ruthven and Pamela Fletcher were finally married at Westminster Abbey on 4 January 1939, with her father, the Reverend Arthur Henry Fletcher, officiating. Their first son, Grey, was born on 26 November 1939.
On the outbreak of the Second World War, Hore-Ruthven was posted to Cairo. Leaving their infant son with her parents in Dublin, his wife followed him to Cairo, where she became friends with Freya Stark and Jacqueline Lampson. She worked in Intelligence with the anti-Nazi Arab Brotherhood of Freedom, and he joined the newly-formed SAS. His wife returned to Ireland in 1942, where she gave birth to their second son, Malise, on 14 May 1942. Promoted to Temporary Major, Hore-Ruthven died in Misurata Italian Hospital in Libya, having been severely wounded in a raid on a fuel dump near Tripoli. He was buried in the war cemetery in Tripoli. He was survived by his wife and two young sons, never having seen either.
After his death, his father was created 1st Earl of Gowrie in 1945, and his widow was styled Viscountess Ruthven of Canberra. She remarried, to Major Derek Cooper, in 1949. On Hore-Ruthven's father's death in May 1955, his elder son, Grey, succeeded as 2nd Earl of Gowrie.
Hore-Ruthven wrote several war poems which were published in Australian and English newspapers. A collection of his poems was published in Australia in 1943 under the title The happy warrior and republished in London in 1944 under the title Desert Warrior: Poems. His collected letters were published in London in 1950 under the title Joy of youth. A memorial fountain was constructed at Government House in Canberra.