Alexander Macomb, Sr. was a prosperous merchant with Loyalist sympathies during the American Revolution. He was born in Belfast, Ireland in 1748 and died in Georgetown, District of Columbia, in 1831. In Detroit, Michigan, during the Revolution, he traded with British and Native Americans, giving them supplies in exchange for furs.

After the war he moved to New York City and became a land speculator and shipping magnate, purchasing tracts of land in Georgia, Kentucky, and North Carolina. In 1788 he built a magnifcent home, which in 1790 became the president's home, occupied by George Washington.

In 1791 he purchased the largest tract yet, from the State of New York, 3,670,715 acres (14,855 km²), since known as "Macomb's Purchase." The tract included much of northern New York, along the St. Lawrence River and eastern Lake Ontario, including the Thousand Islands, at about twelve cents an acre. The purchase was divided into ten large townships. From this purchase are derived the deeds for all the lands that are now included in Lewis, Jefferson, St. Lawrence, and Franklin Counties, as well as portions of Herkimer and Oswego Counties.

The enterprise was a failure; sales of land did not keep pace with the due dates for payments, and Macomb was taken to debtor's prison during the Panic of 1792, over $300,000 in debt. He never regained his fortune.

Six of his sons served during the War of 1812, the eldest, his namesake, was a hero at the Battle of Plattsburg and later was commander in chief of the US army.