Born February 1901 at Newcastle, County Down, Ireland, Sine Mackinnon studied at the Slade School under Tonks from 1918-20 and 1921-4, winning many prizes. In 1924 she left for Paris and studied at La Grand Chaumiere for three years and then the Beaux Arts. During this time she became acquainted with Raoul Dufy, who was so impressed by her work that he introduced her to an art dealer, Barriero, and this let to her first solo exhibition in Paris.
Sine Mackinnon spent much of her life living in France and travelling in Italy and Spain. She came back to England for a period during the war with her husband, Rupert Granviller-Fordham, but returned to Paris shortly after the war ended, later settling in St. Tropez.
Sine Mackinnon had many successful exhibitions in Paris and in London at the Goupil, Alex Reid and Lefevre, Redfern and latterly Tooth galleries. Her work is represented in many public and private collections including the Tate Gallery, London and the Galerie de Jeu de Paume in Paris.