[2] He was the son of Margaret Schiebenstock (1710 – July 21, 1770) and George Michael Hillegass (February 14, 1696 – October 30, 1749), an immigrant from Germany[3] and a well-to-do merchant involved in iron and sugar.

Hillegas was a member of the Pennsylvania Provincial Assembly from 1765 to 1775 and served as treasurer of the Committee of Safety under Benjamin Franklin in 1774.

On July 29, 1775, Hillegas and fellow patriot George Clymer were appointed by the Continental Congress to share the office of Treasurer of the United Colonies.

After Clymer's resignation on August 6, 1776, Hillegas assumed sole ownership of the office, which he held throughout the remainder of the American Revolution, using much of his own fortune to support the cause.

Late in the 19th century, his descendants petitioned[5] to have his portrait appear on the ten-dollar gold certificate in the series issued in the years 1907 and 1922.