Nell was the daughter of Edward Ryan Tenison, a doctor who had been a naval surgeon. With no support from her family, she studied at Cope's School before working as an illustrator. She funded her own further training at Colarossi's Academy in Paris, where she won a medal for drawing, and at the Whistler School.

It was at the Whistler she met the Italian-American Cyrus CUNEO (later ROI), marrying him in 1903.  Both she and Cyrus worked as illustrators in London, Nell contributing to a variety of periodicals including The Girl's Own Annual,The Girls' Realm, The Ladies' Realm, Woman at Home, Black and White, among others, and illustrating books such as A Terrible Tomboy by Angela Brazil.  She worked under both her maiden and married names.

Cyrus tragically died of blood poisoning in 1916 and this affected Nell greatly. She first moved to Dartmoor, then Cornwall, living first at Halsetown and then in St Ives. She bought Down-along House, which she restored, and it became The Copper Kettle (cafe). After 1930 her visits to St Ives became less frequent.

Nell's son Terence CUNEO inherited his mother's talent, attaining great fame as a painter.