 He was one of the 92 hereditary peers elected to keep their seat in the reformed House of Lords under the House of Lords Act 1999.Mowbray was the only son of William Marmaduke Stourton, 22nd Baron Stourton, 26th Baron Segrave, and 25th Baron Mowbray, and Sheila Gully, a granddaughter of William Court Gully, 1st Viscount Selby, who served as Speaker of the House of Commons from 1895 to 1905.He had one sister.