Born in Reigate, Surrey, his father was an ironmonger of St Ives origin. Ninnes studied at West of England College of Art, Bristol, then at the Slade under Tonks and Steer. He married Edyth Mary Day in 1930 and settled in St Ives. Tovey gives him the compliment of being 'an extremely important figure in STISA, serving on the committee from 1932-63, and within that as Vice President and Chairman from 1949-54. He was also President of STIAC in 1936, and in 1953 was founder of the St Ives Society for the Advancement of Music and Arts.

A distinctive artist admired for his "modern" touch in 1930s and 1940s,  his work is sadly neglected now. Initially he worked from Dolphin Studio and made a strong impression on Borlase SMART in 1932. In 1938 he left St Ives and gave an address in Dublin for exhibition purposes. The following year he listed Guildford, Surrey as his base, but then returned to St Ives in 1940, living in the Ayr district.

By 1962, he had shown over 135 works in exhibitions of STISA and RBA but had ceased exhibiting by 1963 due to ill health. Altogether he tackled a wide range of subjects-landscapes, marine paintings, town and harbour scenes, interiors, figure paintings, still life, producing bold and distinctive work in all.