In 1543 the Mercers' Company admitted the 24-year-old Gresham as a liveryman, and later that year he left England for the Low Countries, where, either on his own account or that of his father or uncle, he carried on business as a merchant whilst acting in various matters as agent for King Henry VIII.

In 1544 he married Anne Ferneley, widow of the London merchant Sir William Read, but maintained residence principally in the Low Countries, basing his headquarters at Antwerp[1] in present-day Belgium (then the Spanish Netherlands), where he became renowned for his adept market-play.

Gresham advocated the adoption of various methods – highly ingenious, but quite arbitrary and unfair – for raising the value of the pound sterling on the Antwerp bourse which proved so successful that in just a few years King Edward VI had discharged almost all of his debts.

But Dauntsey's financial operations proved unsuccessful and Gresham was soon reinstated; and as he professed his zealous desire to serve the Queen, and manifested great adroitness both in negotiating loans and in smuggling money, arms and foreign goods, not only were his services retained throughout her reign (1553–1558), but besides his salary of twenty shillings per diem he received grants of church lands to the yearly value of 200 pounds.

Under Queen Elizabeth's reign (1558–1603), besides continuing in his post as financial agent of the Crown, Gresham acted as Ambassador Plenipotentiary to the Court of Duchess Margaret of Parma, Governor of the Netherlands, and was appointed a Knight Bachelor in 1559 prior to his departure.

Gresham's law (stated simply as: "Bad money drives out good") takes its name from him (although others, including the astronomer Nicolaus Copernicus, had recognised the concept for years) because he urged Queen Elizabeth to restore the debased currency of England.

[7] According to ancient legend, the founder of the family, Roger de Gresham, was a foundling abandoned as a new-born baby among long grass in Norfolk during the 13th century and found there by a woman whose attention was drawn to the child by a grasshopper.