Born in Wigan, Lancaster, Garnier was educated at Charterhouse School and originally trained as an engineer. He worked in engineering and technical drawing in Toronto before prospecting for gold in the Yukon.

After studying at Herkomer's school, Bushey (1910) he moved to Newlyn in 1912, studying further under Stanhope FORBES. In 1914 he shared a studio with Richard 'Seal' WEATHERBY at Trewarveneth Farm in Newlyn. Medically discharged from the Army in WWI, he joined the Royal Navy in 1916 and served as a Lieutenant. Throughout the War he kept his Orchard Cottage studio in Newlyn, and subsequently lived there to his death in 1970.

In 1917 he married his cousin Jill BLYTHE (or BLYTH), who was a fellow artist at FORBES SCHOOL. A strong supporter of the Society of Graphic Artists and the Royal West of England Academy, he was better known as an engraver, excelling in aquatints. Alerted to in the latter techniques due to a study of the work of William DANIELL RA, he took this on as his artistic mission in life in addition to the line engravings made direct to copper plate, which formed the basis for the aquatint production. His coloured aquatints of places such as Flushing, Gweek, the Helford River, Lamorna Cove and Polperro were highly regarded.

He was a long-time member of both NAG and STISA, and his writings included several short stories and a novel Bargasoles (1936). During WW II, he worked with the local Home Guard and the local Army Cadet Corps.