James Monroe had long been destined for a life as a lawyer. Both his father and his uncle had pushed the profession since his birth. Thus, three and a half years after leaving college, Monroe found himself back at William and Mary to finish his degree. Simultaneously, he worked with Thomas Jefferson studying law, joining his old friend John Mercer and several other students.

In the spring of 1780, when the capital of the Virginia was transferred to Richmond, Jefferson suggested that Monroe accompany him to the new capital. However, the Revolution again intervened. Jefferson, then governor, received word that a British invasion was possible. He dispatched Monroe to establish a courier route that could move messages 120 a day to give ample warning of any attack; for a month Monroe traveled with the army and spent many hours discussing the future with his colleague and eventual friend, Thomas Pinckney of South Carolina. That assignment completed, he again declined compensation for his time and returned to his small estate in Virginia to continue his law studies. Nevertheless, as the war wound to a close around him, he again found himself longing for a new assignment. His appeals fell on deaf ears, as all of Virginia's regiments had enough officers. As the American army, assisted by French forces, lay siege to Yorktown, Monroe waited impatiently for a part: His final role in the Revolution was to oversee a flag of truce for a British ship delivering tobacco.

With the Revolution settled, Monroe hoped to continue his education abroad and Jefferson urged him to study in London and so presented him with a gift of forty volumes of Parliamentary history. It was not to be, however, for Monroe never found passage to Europe–ending yet another of his dreams.

Luckily, though, Monroe's life began to get on track. In 1782, he won election to the Virginia House of Delegates–taking the seat vacated by his uncle when the latter was appointed to the Continental Congress. In the House, Monroe made such an impression that he was elected to the Governor's Council, which consisted of eight councilors and the governor and was responsible for the day-to-day operations of the state. That same year, Monroe finally earned the right to present cases before the lesser courts of Virginia.

Even in these early forays into politics, Monroe's major issues became visible. He worked to develop the western regions of Virginia, those parts west of the Alleghenies. Monroe felt that westward expansion was vital to the future of the young nation–a feeling helped by the fact that he had been awarded 5,333 acres of land in Kentucky for his military service. Monroe would forever be interested in westward expansion.

After another failed plan to travel to France in 1783, Monroe found himself appointed to represent Virginia in the Continental Congress–perhaps the highest honor bestowed upon a politician at the time. Before leaving for Congress, Monroe spent much of that summer at Jefferson's house, talking with his mentor and learning more about his new life in politics.

In Congress, Monroe took on many different roles as the nation and the Congress struggled with how to form a new, balanced government. Monroe pushed for a stronger government, agreeing with his friend and colleague James Madison. Much of his energy went into dealing with issues of expansion, including developing territorial governments to be used in actions like the Northwest Ordinance of 1787, and became one of the leading opponents of the Jay-Gardoqui proposals, which would have withdrawn the United States's demand of free passage the length of the Mississippi River. In the end, however, Monroe joined with other Virginians like Patrick Henry in opposing the finished Constitution–they disliked the powers given to the Senate.

Monroe returned to Virginia in 1789. While in Congress in 1786 he had married Elizabeth Kortright, the daughter of a wealthy New York merchant; she was not even eighteen when they married.

The Monroes settled in Albemarle County upon their return to Virginia, so that they may be close to Jefferson's estate at Monticello. It was there that the Monroes's two children, both daughters, Eliza and Maria Hester, were to be born. A third child, a son, died in infancy.