Philemon Dickinson managed estates near Trenton after legal study and admission to the bar, although he did not pursue a regular legal practice. During the Revolution he entered New Jersey's provincial government and militia leadership, becoming a brigadier general and then major general commanding the state militia in 1777. His forces supplied local knowledge and manpower alongside the Continental armies.

At Millstone in January 1777 Dickinson led militia against a British foraging force and captured wagons, animals and supplies. His men continued to harass British detachments, patrol routes and respond to raids. During the British withdrawal from Philadelphia in June 1778, New Jersey militia watched and attacked the marching columns in the campaign culminating at Monmouth. Dickinson combined field operations with calling out part-time soldiers to protect farms, roads and communities.

His political connections extended beyond New Jersey. He represented Delaware in the Confederation Congress in 1782–1783 and afterward served on a commission considering a site for the national capital. He also sat in New Jersey's Legislative Council. When William Paterson resigned from the U.S. Senate, Dickinson was elected to complete the term, serving from November 1790 to March 1793. He did not seek another term and returned to the management of his property. He died at The Hermitage near Trenton in 1809. His elder brother John Dickinson pursued a separate career as a political writer.