Being left in the care of a family friend, using his guardian's connections Thornton was educated at Christ's Hospital and at Pembroke College, Cambridge.

[3] He became British vice-consul in Maryland in June 1793 and then served as Envoy Extraordinary and Minister Plenipotentiary to the United States from 1800 to 1804.

He negotiated the Treaty of Kiel for the United Kingdom and was present with the prince royal of Sweden (Jean Baptiste Bernadotte) when the allies entered Paris in 1815.

The title of Count of Cacilhas in the Portuguese nobility was conferred on Thornton and his heirs, for three generations,[6] by the King of Portugal.

Together, they had one daughter and six sons, including: In retirement he lived in Wembury House, Plymouth, Devon, where he died in 1852.