Darlington was elected as a Federalist to the Fifteenth Congress. He declined to be a candidate for renomination in 1818 to the Sixteenth Congress. He was appointed deputy attorney general for Chester County, Pennsylvania in 1820 and became presiding judge of the judicial district comprising the counties of Chester and Delaware from May 1821 until the time of his death in West Chester in 1839. Interment in Friends Burying Ground in Birmingham, Pennsylvania.