Harold was born in England on 13th December 1883; the only son of Sir James Fortescue Flannery, Bt. and Lady Edith (née Jenkyn).
He was educated at Dulwich College, where he played in the 1st XV rugby team of 1901-02. Apart from his love of rugby, he was proficient at lacrosse and represented Surrey in 1903.
He then served an engineering apprenticeship with Humphreys, Tennant & Company, before going up to Trinity Hall, Cambridge University in 1906, where he studied Mechanical Engineering; obtaining his B.A. degree in 1910. He rowed in his college IV at Henley in 1908. He was gazetted second lieutenant in the Essex Royal Horse Artillery (Territorial Army) in 1909, was promoted to lieutenant in the following year, and captain in 1915. He served as a fuze prod officer in Canada, and was awarded an M.B.E. (Military) in 1918.
In 1917, he married Maud (née Boswell) daughter of St. George Boswell of Quebec, Canada. They had a daughter Joan and a son John Derek Fortescue Flannery.
Harold was a director of Flannery, Baggallay & Johnson; Flannery & Gregson; and Flannery & Given. He was president of the Society of Consulting Marine Engineers and Ship Surveyors (1937 - 1939). He was an associate member of the Institution of Civil Engineers, and a member of the Institution of Naval Architects. In 1949 he was elected 3rd Warden of Shipwright’s Company, and was elected Prime Warden in 1952.
In 1943, he succeeded to the baronetcy upon his father's death. Harold died on 19th April 1959; having no surviving male issue, the title became extinct.
[his portrait is illustrated above; courtesy of his nephew, David Lindsay; flanked by the badges of knight bachelor (left) and baronet (right)]