Tatem was born in Appledore, North Devon from where he originally went to sea. He moved to Cardiff at the age of 18 and began work at the shipping offices of Anning Brothers. In 1897 he founded the Lady Lewis Steamship Company with a ship of that name and in 1910 this became the Tatem Steam Ship Company. He became, in addition to that company, chairman of the Atlantic Shipping and Trading company, Dulverton Steamship company, the West of England Steamship Owners' Protection and Indemnity Association, and the British Corporation for Registration of Shipping and Aircraft. He was chairman of the Cardiff Shipowners' Association in 1907 and President of the Institute of Chartered Shipbrokers in 1935. He also came to have directorships in other industries in south Wales and elsewhere. He was chairman of Crosswells Brewery in Cardiff and director of the Cardiff Docks and Railways company, Rhymney Railway company, the Great Western Railway company, Anglo-Equadorian Oilfields and Lobitos Oilfields Ltd, Mount Stuart Drydocks Ltd, and Cardiff Exchange Co Ltd. Tatem was a DL and JP for the county of Glamorgan, of which he became High Sheriff in 1911, as well as becoming a JP for Wiltshire in 1922. He was created a Baronet in 1916 and raised to the peerage as Baron Glanely, of St. Fagans in the County of Glamorgan, on 28 June 1918.
Philanthropy
Tatem was a benefactor of Cardiff University Cardiff and donated the money to build the 1904 Chemistry and Physics block. In 1928 he became an honorary Freeman of the City of Cardiff. He provided £10,000 for the construction of a new church for the Ely district of Cardiff, the Church of the Resurrection, in memory of his wife Ada Mary. The church was consecrated in 1934. He was governor of Cardiff Royal Infirmary and President of the Royal Hamadryad Seamen's Hospital.
During the Spanish Civil War Glanely was part of widespread Welsh support for the Spanish Republic. In 1937 he helped the National Joint Committee for Spanish Relief to open a home at Cambria House, Caerleon for 50 Basquechild refugees. He let the Basque Government charter his two oldest steamships, and, to bring Republican refugees out of the Republican-held part of northern Spain, which was encircled by insurgent forces. On 14 July Molton tried to enter Republican-held Santander but the insurgent cruiser Almirante Cervera captured her and the insurgent armed trawlerGalerna took Molton to Bilbao, which by then had fallen to Franco's forces. Pilton was more successful, firstly carrying many refugees out of Santander, and then on 27 July 1937 bringing refugees from Avilés to France.
Tatem married, on 14 September 1897, Ada Mary, daughter of Thomas Williams of Pengam, Cardiff. The couple had issue one son, Thomas Shandon Tatem, born 20 July 1898, who died on 14 June 1905 aged six years. Lady Glanely died on 10 April 1930. Lord Glanely was killed during World War II in an air-raid in Weston-super-Mare in June 1942, aged 74, when, having no surviving heir, his titles became extinct.