William Mackenzie collected books for the sake of their rarity, age, or beauty. By the time he died in 1828, he had assembled one of the largest private libraries in the United States. It was the most valuable collection of antiquarian books gathered by an American to that point. His assortment of historical material, unprecedented in this country for its depth and breadth, sets Mackenzie apart as the first American rare-book collector. Bequeathed in part in Mackenzie's will to the Library Company of Philadelphia, which then purchased the rest, his library has remained intact for posterity, the legacy of an extraordinary collector.

Mackenzie was born in Philadelphia on 30 July 1758 to Kenneth and Mary Mackenzie. Kenneth Mackenzie had come to Philadelphia by way of Charleston, South Carolina, where his father, also named William, had helped to found a benevolent society for Scottish immigrants in 1730, suggesting that the first William may...