He was given the name of James Cornwallis at birth.

He was educated at Eton College, Windsor, Berkshire, England.

He was educated at St. John's College, Cambridge University, Cambridge, Cambridgeshire, England.

He held the office of Member of Parliament (M.P.) (Tory) for Eye between 1799 and 1806.

He held the office of Member of Parliament (M.P.) (Tory) for Eye between January 1807 and April 1807.

On 9 April 1814 his name was legally changed to James Mann by Royal Licence.

He succeeded to the title of 5th Earl Cornwallis [G.B., 1753] on 20 January 1824.

He succeeded to the title of 9th Baronet Cornwallis [E., 1627] on 20 January 1824.

He succeeded to the title of 9th Baron Cornwallis of Eye, Suffolk [E., 1661] on 20 January 1824.

He succeeded to the title of 5th Viscount Brome, of co. Suffolk [G.B., 1753] on 20 January 1824.

On his death, all of his titles became extinct.