He was born in Germany with the name Wilhelm Rittinghausen. His name as "Willm Rittenhuysen" was on a petition for naturalization of residents of Germantown, Pennsylvania dated 7 May 1691. William was a papermaking apprentice in Germany where he worked at a mill in Broich near the city of Mülheim an der Ruhr. Later he learned the Dutch ways of papermaking while living with his brother in the Netherlands before emigrating to the Pennsylvania Colony in 1688. Two years later he founded the first paper mill to be established in the colonies. This Rittenhouse Mill became the family business for the next century.