Alexandre was born in London in May 1809. He was the youngest son of diplomat Nathaniel Parker
Forth (1744–1809) and Eliza Petrie (1784–1864). His father was King
George III's private envoy to the
Courts of Versailles and Madrid. From his parents' marriage, he had several siblings, including
Frederick Henry Alexander Forth (Lt. Gov. in the
British West Indies and the
Colonial Treasurer of Hong Kong) and Eliza Parker Forth (wife of the Hon. Jabez Henry). After his father's death in April 1809, shortly before his birth, his mother married Alexandre-Jean-Denis Rouen des
Mallets, the Mayor of
Taverny, with whom she had another daughter, Louise Rouen des Mallets (who married Charles Louis, Viscount Terray de Morel-Vindé in 1839), Alexandre's half-sister. His maternal grandparents were Anne ( Keable) Petrie and John Petrie, a senior merchant in
Calcutta in the 1780s. His niece, Edith Ann Mary Forth, married the Scottish tea merchant in
Hong Kong,
Phineas Ryrie. Another niece, Jeanne Marie Terray de Morel-Vindé, married Guy de
Rohan-Chabot,
Pontifical Duke of Ravese. ==Career==