I don’t recall the exact dates but I believe there were no more slaves in the north after 1804. Slavery was also dying in the south at about that time, then the cotton gin came along.
1790—First United States Census Nearly 700,000 slaves live and toil in a nation of 3.9 million people.
1793 Fugitive Slave Act The United States outlaws any efforts to impede the capture of runaway slaves.
1794—Cotton Gin Eli Whitney patents his device for pulling seeds from cotton. The invention turns cotton into the cash crop of the American South—and creates a huge demand for slave labor.
1808 United States Bans Slave Trade Importing African slaves is outlawed, but smuggling continues.
1820—Missouri Compromise Missouri is admitted to the Union as a slave state, Maine as a free state. Slavery is forbidden in any subsequent territories north of latitude 36°30´.
There are documented cases of slavery in Connecticut (went to high school there), Rhode Island and other places in the North as late as 1860...very small pockets.