He studied science at City of London School, Central Technical College of the City, and Guilds of London Institute.
Renwick began professional research work with the Leathersellers Company, and in 1898 became a research chemist at Britannia Works Company Ltd, forerunner of Ilford. In 1922 he was hired by Dupont to direct the photographic research department at their Parlin, New Jersey, plant. He returned to England in 1925, rejoining Ilford and becoming research director there in 1930.
His earliest contributions to photographic technology were studies of light-absorbing and -scattering media. In 1912 he improved W. B. Ferguson's densitometer, named the Ferguson-Renwick bar photometer. With D. F. Benson's assistance in 1918, he further improved it, changing it from a two-lamp device to one lamp with mirrors. Before World War I, Renwick studied the underexposure curve of the Hurter and Driffield curves, pointing out that it was of greater significance than previously believed. During the war his researches on panchromatic emulsions led to the production of outstanding plates for aerial photography. While at Dupont he developed positive motion film emulsion.