Key was a lawyer in Maryland and Washington, D.C. for four decades and worked on important cases, including the Burr conspiracy trial, and he argued numerous times before the Supreme Court.

Key owned slaves from 1800, during which time abolitionists ridiculed his words, claiming that America was more like the "Land of the Free and Home of the Oppressed".

[13] He graduated from St. John's College, Annapolis, Maryland, in 1796 and read law under his uncle Philip Barton Key who was loyal to the British Crown during the War of Independence.

[18] Key and Colonel John Stuart Skinner dined aboard HMS Tonnant on September 7, 1814, following the Burning of Washington in August.

Beanes was accused of aiding the detention of several British Army stragglers who were ransacking local homes in search of food.

Key was unable to do anything but watch the 25-hour bombardment of the American forces at Fort McHenry during the Battle of Baltimore from dawn of September 13 to the next morning.

[20][21][22] At dawn, Key was able to see a large American flag waving over the fort, and he started writing a poem about his experience on the back of a letter that he had kept in his pocket.

His untitled and unsigned manuscript was printed as a broadside the next day under the title "Defence of Fort M'Henry", with the notation: "Tune – Anacreon in Heaven".

This was a popular tune that Key had already used as a setting for his 1805 song "When the Warrior Returns", celebrating American heroes of the First Barbary War.

The song was finally adopted as the American national anthem more than a century after its first publication by Act of Congress in 1931, signed by President Herbert Hoover.

He assisted his uncle Philip Barton Key in the sensational conspiracy trial of Aaron Burr and in the expulsion of Senator John Smith of Ohio.

[24] In 1829, Key assisted in the prosecution of Tobias Watkins, former U.S. Treasury auditor under President John Quincy Adams, for misappropriating public funds.

He also handled the Petticoat affair concerning Secretary of War John Eaton,[25] and he served as the attorney for Sam Houston in 1832 during his trial for assaulting Representative William Stanbery of Ohio.

[32] Key is known to have publicly criticized slavery's cruelties, and a newspaper editorial stated that "he often volunteered to defend the downtrodden sons and daughters of Africa."

[35] Key was a founding member and active leader of the American Colonization Society (ACS), whose primary goal was to send free black people to Africa.

[36] The ACS was not supported by most abolitionists or free black people of the time, but the organization's work would eventually lead to the creation of Liberia in 1847.

Led by newspaper editor and publisher William Lloyd Garrison, a growing portion of the population noted that only a very small number of free black people were actually moved, and they faced brutal conditions in West Africa, with very high mortality.

Lundy's article, Key said in the indictment, "was intended to injure, oppress, aggrieve, and vilify the good name, fame, credit & reputation of the Magistrates and constables" of Washington.

It accused Crandall of "seditious libel" after two marshals (who operated as slave catchers in their off hours) found Crandall had a trunk full of anti-slavery publications in his Georgetown residence and office, five days after the Snow riot, caused by rumors that a mentally ill slave had attempted to kill an elderly white woman.

In an April 1837 trial that attracted nationwide attention and that congressmen attended, Key charged that Crandall's publications instigated slaves to rebel.

[39] Key, in his final address to the jury said: Are you willing, gentlemen, to abandon your country, to permit it to be taken from you, and occupied by the abolitionist, according to whose taste it is to associate and amalgamate with the negro?

He was described as a "devoted and intimate friend" of Bishop William Meade of Virginia, and his "good literary taste" was credited for the quality of the church's hymnal.