He was born in America, and was a pupil of the venerable Benezet, who took pains very early to interest his feelings on this great subject.

Benezet employed him occasionally, I mean in a friendly manner, as his amanuensis, to copy his manuscripts for publication, as well as several of his letters written in behalf of the cause.

This gave his scholar an insight into the subject; who, living besides in the land where both the Slave Trade and slavery were established, obtained an additional knowledge of them, so as to be able to refute many of those objections, to which others, for want of local observation, could never have replied.
In the year 1772, Anthony Benezet introduced William Dillwyn by letter to several of the principal people of Carolina, with whom he had himself corresponded on the sufferings of the poor Africans, and desired him to have interviews with them on the subject.

He charged him also to be very particular in making observations as to what he should see there.

This journey was of great use to the latter, in fixing him as the friend of these oppressed people; for he saw so much of their cruel treatment in the course of it, that he felt an anxiety ever afterwards, amounting to a duty, to do every thing in his power for their relief.
In the year 1773, William Dillwyn, in conjunction with Richard Smith and Daniel Wells, two of his own society, wrote a pamphlet in answer to arguments then prevailing, that the manumission of slaves would be injurious.