William Paca worked in a law office in Annapolis, where he formed a friendship with another young lawyer, Samuel Chase.
They were more than just colleagues; they shared an opposition to the Stamp Act (1765). This common thread inspired them to organize the Annapolis Sons of Liberty.
Paca was elected as a delegate to the 1st and 2nd Constitutional Conventions (1774–1776). He would sign the Declaration of Independence, pledging his life, his fortune and his sacred honor.
After the death of his wifein 1774, Paca fathered two children out of wedlock: Hester (1775–1793), whose mother Levina was a free woman of color, and Henrietta Maria (1777–1850) the daughter of Sarah Joice. His daughter Henrietta Maria became the third wife of Thomas Grafton Addison.
In 1777, Paca married Anna Harrison White and they had one child, Henry (1778–1781).
Paca served in the Maryland Senate (1777–1780) and as a judge on the Admiralty Court (1780–1782) and as Governor of Maryland (1782–1784).
He served on the U.S. District Court from 1789 until his death, on October 23, 1799. He was buried in the family graveyard on Wye Plantation, Queen Anne’s County, Maryland.