Garrett's parents were members of the Society of Friends (Quakers), and he remained a member throughout his life...His work as an abolitionist and Underground Railroad organizer and operator would result in his national reputation.
While most northern Quakers were moderate in their antislavery views, emphasizing primarily their own rejection of slavery, Garrett went much further, becoming a follower of Boston abolitionist William Lloyd Garrison, who offended some Quakers with his strong language and confrontational style. Like Garrison, Garrett advocated nonviolent resistance to slavery. Working with abolitionists in the Philadelphia area, Garrett organized a network of sympathizers who provided funds, transportation, and sometimes hospitality to slaves who had fled from their masters. During his lengthy involvement with the Underground Railroad, he kept records of 2,700 slaves he had helped escape. His work, along with that of Levi Coffin in Cincinnati, contributed to the perception that a well-organized escape route for slaves extended throughout the nation.
Even though Delaware was a slave state, except for several newspaper attacks Garrett experienced remarkably little opposition to his Underground Railroad activity, especially in the years just prior to the Civil War. In 1856 Garrett wrote a friend: "There is about as much anti-slavery feeling here as in Boston, and quite as freely expressed."