William Coleman was born and died in Philadelphia, where he was educated and studied law. His parents were Quaker; his mother, Rebecca, had arrived in the new colony of Pennsylvania as a child in 1683 and his father, also William Coleman, was a carpenter. Young William Coleman married Hannah Fitzwater in 1738; the couple was childless, but Coleman adopted his nephew George Clymer.
After he was admitted to the bar, Coleman held a variety of municipal offices, beginning as town clerk and clerk of the City Court. He became a judge of various local courts including the Orphan’s Court, Court of Common Pleas, and Quarter Sessions. In 1758 he was appointed an Associate Justice of the Supreme Court of Pennsylvania. He was also a merchant, in partnership with Thomas Hopkinson. He was also active in Philadelphia’s emerging cultural institutions. By 1727 Coleman was a friend of Benjamin Franklin and member of Franklin’s Junto. He was a founder and first treasurer of the American Philosophical Society, one of the first directors of the Philadelphia Contributionship, and an early supporter of Pennsylvania Hospital.
Coleman was also a founder of the Academy and College of Philadelphia, serving as the original clerk of the Board of Trustees, from 1749 to 1755, and as its first treasurer, from 1749 to 1764.