George Washington Williams was born free in 1849 in Bedford Springs, Pennsylvania, to two African Americans. During the American Civil War, Williams ran away to enlist at the age of 14 in the Union Army under an assumed name; he fought during the final battles.

After the war, Williams went to Mexico, where he was among Americans who joined the Republican Army under the command of General Espinosa, fighting to overthrow Emperor Maximilian. In the late 1880s, Williams turned his interest to Europe and Africa. After having been impressed by meeting King Leopold II of Belgium, he traveled in 1890 to the Congo Free State (then owned by the king) to see its development.

Shocked by the widespread brutal abuses and slavery imposed on the Congolese, he wrote an open letter to Leopold in 1890 about the suffering of the region's native inhabitants at the hands of the king's agents. This letter, which subsequently popularized the term "crimes against humanity", was a catalyst for an international outcry against the regime running the Congo, which had caused millions of deaths.

This was an early but not, as is often claimed, the first use of the term in its modern sense in the English language. In his first annual message in December 1889, U.S. President Benjamin Harrison spoke about the slave trade in Africa as a "crime against humanity". Already in 1883, Williams had used the same term in his reflections about slavery in the United States.

Williams seems like a really cool guy, after his letter the term begun to be used during the Hague Convention.