From the onset of European settlement in North America slavery was a recognized institution and integral to the colonial economy. Although Quakers discouraged the practice, settlers of other religious faiths living in the Delaware Valley maintained and relied heavily on the systems of bondage and indentured servitude to improve the land, raise crops and livestock, and generally support the colony's social structure. A census of New Jersey in 1726, recorded roughly 2,500 persons as being non-white, the vast majority of these individuals being enslaved Africans and African-Americans, yet also comprising members of other groups, including Native Americans.
Slavery persisted throughout the 18th century with the number of slaves in New Jersey reaching almost 11,500 by 1790, although sentiment against this inhumane practice gradually began to build during and after the Revolution. Slaves were bought and sold, hired out and frequently forced to work and live under appalling conditions. Their liberties were greatly restricted and they were forbidden to meet in large groups. Slaveholding was more prevalent in the southern counties of New Jersey among Dutch-American settlers and landowners with interests in the West Indies where plantations with large slave populations were common.
In 1778, William Livingston, Governor of New Jersey, freed all of his own slaves, condemning the practice of slavery as "odious and disgraceful." Abolitionist views slowly received a wider hearing and in 1792 an Abolition Society was formed in New Jersey. In 1804 the State passed an "Act for Gradual Abolition of Slavery" which guaranteed freedom to slaves born after this year at a certain age (females at age 21 and males at age 25). Eight years later, echoing the nationwide prohibition of slave trading enacted in 1808, New Jersey finally banned the export of slaves out-of-state. However it was not until 1865, at the conclusion of the Civil War and after passage of the 13th Amendment to the U.S. Constitution, that New Jersey - the last of the northern states to do so - finally made the practice of slavery illegal.
Links to learn more - John Woolman House, Mount Holly; William Trent House, Trenton; Peter Mott House, Lawnside; New Jersey State Archives, Trenton