Joseph Sim was born in Prince George’s County, Maryland, around 1725 or 1728. He was a son of Patrick Sim and Mary Brooke Sim and grew up among interrelated Catholic and Anglican planter families along the Potomac and Patuxent watersheds. The exact year of his birth varies in genealogical reconstructions, and little direct testimony survives about his childhood.

No reliable source identifies a school, tutor, or apprenticeship for Sim. His later work nevertheless required literacy, accurate handwriting, knowledge of legal forms, and skill with accounts. These abilities may have been learned at home, through local instruction, or in a law office, but choosing among those possibilities would be speculative. His family position would also have familiarized him with tobacco planting, land ownership, credit, and county government. Such prosperity rested in part on enslaved labor.

By 1749 Sim had become clerk of the Prince George’s County Court, a significant office for a man still in his twenties. The clerk recorded deeds, wills, judgments, licenses, and other proceedings; maintained the county’s documentary memory; collected fees; and served lawyers, magistrates, planters, merchants, and ordinary litigants. Holding the post confirms that Sim had already acquired the trust and technical competence that his sparse early biography otherwise leaves undocumented.

Sim also entered the county militia and held the rank of major by the early 1750s. Militia standing reflected both social position and the ability to organize men, supplies, and communications. Before 1754 he married Catherine Murdock, daughter of the merchant James Murdock. The match joined him to another influential Prince George’s County family and to commercial networks centered on Upper Marlboro.

During the 1750s and 1760s Sim combined court administration, landholding, trade, and militia responsibility. He maintained a storehouse at Upper Marlboro by about 1765 and remained court clerk until 1767. These years mark the transition from his poorly documented youth to a clearly visible public career. Whatever the precise form of his education, legal recordkeeping and county commerce—not a documented college course—provided the practical preparation for his later service in Maryland politics.