Lewis was born in Llandaff, Wales, and educated at the prestigious Westminster School in London before immigrating to the North American colonies at 22 years old. Lewis founded businesses in New York and Philadelphia, winning lucrative contracts to provision the British army in America.
During the Seven Years' War, Lewis served General Hugh Mercer as an aide. Traveling with Mercer, Lewis was captured in Oswego and taken as a prisoner of war to France. For his service, the New York government granted him 5,000 acres upon his return to the colony.
In 1765, Lewis served as a delegate to the Stamp Act Congress and retired from his mercantile career to settle in Whitestone, Long Island, becoming active in revolutionary politics, including the Sons of Liberty. In 1775 alone, he participated in the provincial convention, the Continental Congress and the Committee of One Hundred. In 1776, he continued to serve the first two bodies and signed the Declaration of Independence. He continued as a member of the Continental Congress until 1779, when he assumed the position of commissioner of the Board of Admiralty.
Lewis' patriotism came at a high cost. The British army destroyed his Long Island estate and took his wife prisoner in 1776. She remained a prisoner, reportedly without a bed or change of clothes, for many months because the Continental Army had no women of similar status in captivity for whom George Washington could orchestrate an exchange. Neither Mrs. Lewis' health nor the couple's fortune ever recovered from the attack.