Alexander was one of three children. Soon after the family moved to Albany he formed a lifelong friendship with a boy in nearby Schenectady by the name of William Constable.
Sometime after 1772, Macomb married Catharine Navarre, a direct descendant of King Henry IV of France. The couple raised ten children, four sons and six daughters. One of the boys, Alexander Jr. (1782-1841), was later named a General, and commanded the victorious land forces at the Battle of Plattsburg during the War of 1812. General Macomb himself had a son, Commodore William H. Macomb (1819-1872), who served with distinction as a Union officer during the Civil War. During World War II, a destroyer (DD-458) was named for him.
In 1785, with the fortune his family had made in the fur trade, Alexander Macomb moved to New York City where he built an elegant mansion near Trinity Church at Broadway and Wall Street. When George Washington became president in 1789, he rented the house from Macomb for the duration of his stay in the city. Catharine Navarre Macomb died that same year.
In 1791, Macomb remarried, to Jane Rucker, the widow of John Rucker, another friend of William Constable. This second marriage produced seven more children, three sons and four daughters, making a total of seventeen. One of the sons of this second marriage, John, married Christina Livingston whose father, Phillip Livingston was one of the signers of the Declaration of Independence.
While living in New York City, Alexander Macomb became associated with the famed New York Patriots and new national leaders Alexander Hamilton, Daniel McCormick, and Governeur Morris, who had helped draft the Constitution. In 1792, at the very peak of his financial and social success, Macomb became involved in what would later be called a Ponzi Scheme, which, when it failed, besmirched his previously unstained reputation.
That same year, the financially struggling State of New York decided to sell land which it had seized from the pro-British and now-defeated Iroquois. Macomb, Constable, and Daniel McCormick, purchased nearly 4,000,000 acres (1,600,000 hectares) of North Country land from the State at the extremely low price of just 8 pence per acre in the devalued New York State currency of the time.This was an enormous amount of land, about one-eighth of the entire State of New York, and it cost the three men a fraction of what it should have. Known ever since as "Macomb's Purchase" It remains the largest, cheapest land deal in New York State history.
In the shadow of the failed Ponzi Scheme, and convinced that something illegal had occurred, the New York State Legislature held exhaustive Hearings into the land purchase, which further damaged Macomb's reputation. He also could not sell the land during the pending proceedings.
Ultimately, no wrongdoing was uncovered, but Macomb had badly miscalculated. Although the land had been cheap, it had not been free, costing him about $300,000.00, and the delay occasioned by the Hearings in selling it hurt him financially. When he finally could sell it, he found relatively few buyers. The land was considered worthless and barren wilderness, and the harsh winters deterred settlers. Lastly, the mere fact that there had been Hearings, combined with the fallout from the Ponzi Scheme, left a cloud overhanging the land which made prospective buyers shy away from Macomb. He ended up in debtor's prison.
In order to make good his debts, he sold much of the land to his friend, Daniel McCormick. Whether this was an arm's-length transaction, a hostile takeover, or a straw man deal is unknown, but it helped Macomb recoup his losses.
In just a few years, he apparently managed to regain some of his wealth. Turning his back on the North Country, Macomb returned to New York City. In 1798 he purchased 100 acres in the Bronx across the Harlem River from the northern tip of Manhattan and here he built a fine stone mansion.
In 1821 Macomb, now aged 73, went to live with his son, General Alexander Macomb Jr., in Georgetown, D.C.. After his victory in the War of 1812, the younger Macomb had advanced to become Commander-in-Chief of the U.S. Army, and he invited his aging parents to come live with him.
Alexander Macomb died at Georgetown in 1831, but "Macomb's Purchase" lives on forever. Today his name appears in virtually all of the property deeds being recorded in the North Country.