Immediately upon graduating he accepted an offer from the office of one of New England’s most gifted law professor’s, devoting much of his energy there. In 1780, after being admitted to the bar he quickly established a large and lucrative practice outside Newburyport, Massachusetts that continued to flourish throughout his life. King’s first dabble with politics was when he was elected to the Massachusetts General Court in 1783. Massachusetts officials saw his exceptional value and decided to then send him to the Confederation Congress from 1784 through 1787. In 1789 and again in 1795 Rufus was elected New York State Assembly. This was made possible due to the great support and encouragement from his friend, Alexander Hamilton, and fellow supporter, Governor George Clinton. King played a key diplomatic role as Minister to the Court of St. James from 1796 to 1803, and again from 1825 to 1826.
King was a man of great integrity and believed in equality for all beings, which explained his long history of opposition to the expansion of slavery and the slave trade. Rufus’ stand was a combination of the product of his moral conviction, which coincided with the political realities of New England federalism. He was so dedicated to the cause that in 1817 he supported Senate action to abolish domestic slave trade. Furthermore, in 1819 he was a lead speaker for the antislavery amendment to the Missouri statehood bill. The admirable traits that earned King much recognition and assisted him in his life long political journey were a combination of the example led by each of his two brothers; William King whom was the first governor of Maine and his other brother, Cyrus King, who was a U. S. Representative from Massachusetts. In 1825, John Quincy Adams once again requested Rufus’ assistance to rule as Minister of Britain. Unfortunately, due to his rapidly declining health a year later he regrettably had to resign and returned to New York. Rufus King died shortly after his return home on April 29, 1827. He was laid to rest at Grace Episcopal Church Cemetery in Jamaica, New York.