His great-grandfather, grandfather, father, and five brothers served as army officers, but Denison joined the Navy in 1867, as a midshipman.

From October 1900 to November 1902 Denison was in command of the protected cruiser HMS Niobe, serving in the Channel Squadron.

In March 1901 Niobe was one of two cruisers to escort HMS Ophir, commissioned as royal yacht for the world tour of the Duke and Duchess of Cornwall and York (later King George V and Queen Mary), from Spithead to Gibraltar,[6] and in September the same year she again escorted the royal yacht from St Vincent to Halifax, Nova Scotia.

She took part in the fleet review held at Spithead on 16 August 1902 for the coronation of King Edward VII,[7] and the following month visited Souda Bay, Crete for combined manoeuvres with other ships of the Channel and Mediterranean stations.

From July 1903 to September 1904 he was in command of the pre-dreadnought battleship HMS Montagu in her first commission in the Mediterranean Fleet, after which he was Captain Superintendent of Pembroke dockyard for two years from 1 October 1904.