The young Mifflin's conscience was troubled as he came to understand that the children he had played with growing up on his father's plantation were slaves.
He "determined never to be a slave-holder,"[8]: 78 but in 1767, he married Elizabeth Johns (c. 1749–1786), and her family provided them with a plantation in Kent County, Delaware and several slaves as a dowry.
Thomas Clarkson, an English abolitionist, wrote that Mifflin "was the first man in America to unconditionally emancipate his slaves.
[6]: 38–53 During the American Revolutionary War, Warner Mifflin also became a leading Quaker peace activist, despite the danger of being associated with loyalism for doing so.
He traveled several thousand miles on horseback through most of the Mid-Atlantic states and New England to promote his anti-war message on behalf of his fellow Quakers.
[1][3]: 40, 74–75 His refusal to pay any taxes that would support the war effort resulted in seizures of part of his property by sheriffs.
[6]: 54–92 Mifflin expanded the abolition campaign beyond what even most Quakers were likely to support, to as a pioneer in the idea that freed ex-slaves should receive reparations (or "restitution"), in the form of cash payments, land or shared crop arrangements.
He also arranged tours of groups of former slaves into plantation areas to advertise the successes of free blacks, in order to discredit the anti-abolitionist argument that freed people would not work.
He also began trying to halt the domestic slave trade, and to stop the kidnapping of free blacks to enslave them in other states.
Mifflin answered by publishing "A Serious Expostulation with the Members of the House of Representatives of the United States" (1793), which challenged the moral conscience of the congressmen.
He believed not only that slavery should be abolished, but that African Americans wanted nothing more than a level playing field to demonstrate their natural equality with white people.
[5] President John Adams expressed sympathy with Mifflin's aims in response to having been sent a pamphlet written by him, however took a view that slavery must be abolished gradually.
She was the first woman to advocate the project for African colonization of former slaves, and one of the earliest proponents of the cultural assimilation, rather than removal, of Native Americans.