William Paca was born on October 31, 1740, at his family’s plantation near Abingdon in Baltimore County, now Harford County, Maryland. He was a son of John Paca, a prosperous planter and landowner, and Elizabeth Smith Paca. The family’s wealth rested on agriculture and enslaved labor. William grew up among several siblings in a household able to provide opportunities beyond those available to most Maryland children.

In 1752, when William was about eleven, he and his brother Aquila were sent to the Academy and Charity School of Philadelphia. Living away from rural Maryland placed him in the largest city of British North America and exposed him to its diverse religious, commercial, and intellectual life. Beginning in 1756 he continued at the College of Philadelphia, now the University of Pennsylvania. He studied the classical and liberal curriculum of the period and received his bachelor’s degree in May 1759. He later obtained the customary master’s degree in 1762.

Paca chose the law. After college he moved to Annapolis and read law under Stephen Bordley, one of Maryland’s leading attorneys. He also enrolled at the Middle Temple in London, a prestigious English Inn of Court, though it is uncertain how much time he personally spent there. Legal study trained him in English common law, property, procedure, and constitutional argument. Annapolis also introduced him to a rising generation of Maryland lawyers, including Samuel Chase, with whom he formed a close political partnership.

In 1763 Paca married Ann Mary Chew, whose family connections linked him to prominent Maryland and Pennsylvania circles. He was admitted to practice and began building his career in Annapolis. Marriage, professional qualification, and urban connections completed his emergence from a planter household into the colonial legal elite. Soon afterward, the Stamp Act controversy drew Paca and Chase into organized opposition to parliamentary taxation.