Tomkinson joined the Royal Navy in 1891 and served in the destroyer during the Boxer Rebellion in 1900. He served in World War I initially commanding the destroyer and seeing action at the Battle of Heligoland Bight in 1914 and the Battle of Dogger Bank in 1915; his war service continued as Senior Naval Officer, British submarines in Venice in 1915 and as Commander of the light cruiser in 1916 before seeing action again during the Zeebrugge Raid and the Ostend Raid in 1918.