By 1768, Harrison had a steady practice; after meeting Washington, Harrison represented him on several legal matters. In the years prior to the American Revolution, Harrison became identified with the Patriot cause in Virginia; he supported the Virginia Nonimportation Resolves in 1770, and served as the clerk for the Fairfax Resolves in 1774. Harrison was also a member of Alexandria's Committee of Correspondence. Additionally, he joined the militia and served as an officer in the Fairfax Independent Company in 1774 and 1775. In September 1775, Harrison was commissioned as a major in the Fairfax County Militia.
American Revolution
On November 6, 1775, Robert Hanson Harrison was appointed an aide-de-camp to General Washington. The following May, Harrison was appointed military secretary to General Washington in lieu of Joseph Reed. The Continental Congress approved his commission as a lieutenant colonel on June 5, 1776. In this capacity, he assisted with the drafting, writing and organization of the Commander-in-Chief's voluminous correspondence for almost five years. Moreover, President James Monroe testified that "in all the actions in which General Washington commanded, Colonel Harrison was present, near the person of the General." Along with Alexander Hamilton, Robert Hanson Harrison negotiated for the exchange of prisoners of war. The death of his father, Richard Harrison, in 1780 began a chain of events that led to Robert Hanson Harrison's resignation from the Continental Army in the spring of 1781. George Washington attested that Robert Hanson Harrison discharged his duty "with conspicuous abilities – That his whole conduct during all the interesting periods of the war has been distinguished marked by the strictest integrity and the most attentive & faithful services while by personal bravery he has marked his conduct upon many occasions been distinguished on sevl occasions"
Later career
Returning to Maryland, Robert Hanson Harrison was appointed Chief Justice of the General Court of Maryland on March 12, 1781. During the war, Harrison's court considered property confiscation cases for individuals accused of disloyalty to the United States. Despite being considered a highly capable judge, he declined appointments to higher courts because of failing health, including Chancellor of Maryland. When the federal government was established, President George Washington nominated Harrison to the Supreme Court of the United States as an Associate Justice on September 24, 1789. The United States Senate confirmed his appointment two days later. Harrison declined the appointment, citing familial responsibilities and his health as two prime reasons. Washington and Hamilton urged Harrison to reconsider. Washington's friend Dr. James Craik urged him to send Harrison a personal appeal, believing "A Letter from you…would I am convinced act more powerfully upon him than all the Anodynes in an Apothecarys Shop". Harrison agreed to accept, and began the trip to New York City, then the temporary national capital. His health worsened, and he returned home to Bladensburg, Maryland. In a January 21, 1790 letter to Washington, Harrison informed Washington that he was unable to make the trip and again declined the appointment.
Death and burial
Harrison died at his home in Charles County on April 2, 1790. His burial location is not known: He may have been interred at his father's Walnut Landing estate, at the Durham Parish church where his brother was rector, or at the Episcopal church cemetery in Port Tobacco.
Family
Harrison's first wife was Sarah Johnston, a daughter of George Johnston Sr., a prominent Virginia attorney and political leader. After her death, he married Grace Dent of Charles County, Maryland.
