Watson was a member of the original committee of the Corporation of Lloyds of London in 1772 and later served for ten years as its chairman.

He combined his mercantile affairs with government business.
He visited Massachusetts, New York and other colonies before the American Revolutionary War, during which time he intercepted letters to General Thomas Gage that were said to prove that Gage was a spy.
 Calling him ‘a traitor’, William Dunlap wrote that Watson "ingratiated himself with many leading Americans, obtained as much information on their designs as he could, and transmitted it to his chosen masters."

In November 1775 Watson accompanied the American prisoner Ethan Allen on a voyage from Canada to England.
Allen wrote that he "was put under the power of an English Merchant from London, whose name was Brook Watson: a man of malicious and cruel disposition, and who was probably excited, in the exercise of his malevolence, by a junto of tories, who sailed with him to England ..."
Watson was examined by the House of Commons in 1775, when Lord North's bill concerning the fisheries of New England was before parliament.
 In 1782 he acted as Commissary General to the army commanded in North America by Sir Guy Carleton.

Brook Watson & Co owned at least two South Sea whaling ships.
These were Prince William Henry (1792–1795) and Brook Watson (1802–1810).
Watson was a member of the original committee of the Corporation of Lloyds of London in 1772 and later served for ten years as its chairman.
He combined his mercantile affairs with government business. He visited Massachusetts, New York and other colonies before the American Revolutionary War, during which time he intercepted letters to General Thomas Gage that were said to prove that Gage was a spy.  Calling him ‘a traitor’, William Dunlap wrote that Watson "ingratiated himself with many leading Americans, obtained as much information on their designs as he could, and transmitted it to his chosen masters."
In November 1775 Watson accompanied the American prisoner Ethan Allen on a voyage from Canada to England. Allen wrote that he "was put under the power of an English Merchant from London, whose name was Brook Watson: a man of malicious and cruel disposition, and who was probably excited, in the exercise of his malevolence, by a junto of tories, who sailed with him to England ..."
Watson was examined by the House of Commons in 1775, when Lord North's bill concerning the fisheries of New England was before parliament.  In 1782 he acted as Commissary General to the army commanded in North America by Sir Guy Carleton.
Brook Watson & Co owned at least two South Sea whaling ships. These were Prince William Henry (1792–1795) and Brook Watson (1802–1810).
Brook Watson