There were many American movements for the Abolition of slavery earlier than the 19th century (in the 18th century). There were a few abolitions, elsewhere in the world, earlier than that.
In 1777, the constitution of the Republic of Vermont freed all adult slaves.
In 1780, Pennsylvania passed an act for the gradual abolition of slavery.
In 1783, the Massachusetts Supreme Court ruled slavery unconstitutional in Massachusetts (according to the 1780 Massachusetts Constitution).
In 1783, New Hampshire began a gradual abolition of slavery.
In 1784, Connecticut and Rhode Island began gradual abolitions of slavery.
In 1787, the US Congress prohibited slavery in the Northwest Territories.
In 1799 New York state passed a gradual abolition of slavery.
In other parts of the world:
In 1117 Slavery was abolished in Iceland.
In 1215, the Magna Carta was signed in England. clause 30 provided the basis for slavery becoming illegal in England under English Common Law.
By 1274, slavery had been abolished in Norway.
In 1315, King Louis X abolished slavery in France.
In 1335 slavery was abolished in Sweden.
In 1347 slavery was abolished in Poland.
In 1588 Lithuania abolished slavery.
In 1590 slavery was banned in Japan.
In 1723 Russia abolished slavery but retained serfdom.
In 1761, Portugal abolished slavery in Portugal and Portuguese India (but not in Brazil, Africa, and other Portuguese possessions).
In 1794 France abolished slavery in all its possessions (though this was rescinded in 1802).