Dickinson's education began under the tutorship of William Killen afterward chancellor of Delaware, at that time a law student in Judge Dickinson’s office. He then entered the College of Philadelphia (now the University of Pennsylvania), from which he was graduated in 1739.
For some time prior to his father’s death in 1760 Dickinson managed the latter’s enormous estates in Kent County, Delaware, United States and in Talbot, Dorchester, and Queen Anne counties, Maryland, United States. He then settled in Philadelphia, where he read law with his brother John, whom he later joined in signing the non-importation agreement of November 1765.
When New Jersey voted to raise ten battalions of infantry in 1775, Philemon, who had an estate near Trenton, was appointed colonel of the Hunterdon County Battalion, and in October was commissioned brigadier-general of New Jersey militia.
The following year he was elected to the Provincial Congress of New Jersey, but he was not a member, as is alleged, of the committee that drafted the New Jersey constitution of 1776.
In January 1777 Dickinson, with 400 raw militia, defeated a foraging expedition sent out by Cornwallis to capture flour stored in a mill near Somerset Court House on the Millstone River, and captured forty wagons and a hundred draft horses. In reporting the engagement to the president of Congress Washington warmly commended Dickinson. In June, Gov. William Livingston appointed him major-general and commander-in-chief of the New Jersey militia.
In November Dickinson suggested that he himself descend upon Staten Island while Gates and Putnam attack Long Island and New York. Washington disapproved the proposed attack on Long Island, but thought the general plan “of counteracting the intended reinforcements for Mr. Howe’s army, by a demonstration of designs upon New York . .. an exceedingly good one. ” Dickinson accordingly landed on Staten Island, which was occupied by the Loyalist regiments of Gen. Cortland Skinner and the Waldeckers of Gen. Campbell, both of whom, warned of Dickinson’s plans, narrowly escaped.
In the spring of 1778 Clinton, having evacuated Philadelphia, began retreating across the Jerseys toward New York. Dickinson and Gen. William Maxwell, having been ordered by Washington to obstruct the roads and harass the British, “destroyed every bridge on the road, ” as Clinton reported, in a country abounding in small streams and marshes. The delay enabled the Continental Army to come up with the British. Then came the battle of Monmouth, following which victory Washington, in his order to the troops, thanked Dickinson and the New Jersey militia.
In July Dickinson acted as second to Gen. John Cadwalader in his duel with Thomas Conway. He was for a short time chief signal officer of the middle department (1778 - 79). For his “spirited attack” at the battle of Springfield in June 1780 he received the praise of Gen. Greene. Defeated for governor by William Livingston in 1778, 1779 and 1780, he was appointed in 1781 commissioner of the newly created state loan office, and the following year was elected to Congress from Delaware, where he was a property owner.
Elected to the New Jersey Council, he served two terms as vice-president of that body (1783 - 84). In 1785 Congress appointed him, Robert Morris, and Philip Schuyler commissioners to select a site for a Federal capital. The question was not settled, however, until the adoption of the Constitution. Defeated for United States senator by William Paterson, Dickinson was later elected to fill Paterson’s unexpired term (1790 - 93) when the latter succeeded Livingston as governor.
Dickinson was married twice: on July 14, 1767 he married Mary Cadwalader, daughter of his uncle, Dr. Thomas Cadwalader, and, after her death, her sister Rebecca.