Jschooltiger, et al, is certainly right about the decline of slavery in the North heading into the 19th century, but that still leaves the fact that the northern parts of the US, and the colonies that preceded the nation, included slavery in their economies for centuries prior to its general decline heading into the 19th century. We're not talking about a dozen unfortunate victims of slavery either. In 1740 Philadelphia, for example, as much as 15% of the workforce was enslaved. Massachusetts, New York, and Rhode Island merchants took active part in the slave trade. And it doesn't get talked about much, so why is that?
The reason for this, mainly, is because there's a major difference between what historian Ira Berlin has called "slave societies," in which slavery forms the cornerstone of nearly all labor and production in the economy, and "societies with slaves," where slavery exists but is just one of multiple labor systems and doesn't dominate production within the economy. As you might guess, many parts of the South became slave societies whereas the North generally remained simply a society with slaves.
OK, but what does that mean? It means that in the North slavery, historically speaking, doesn't matter nearly as much. It didn't have the same transformative effect on ideology, society, and the economy that slavery had in the South. Slavery never really had this chance in the North for a variety of reasons including geography/climate, economic diversification, competition from other labor sources, and some ideological difference. This meant that Northern societies developed in directions incompatible with slavery and, because they had never relied upon it for their primary livelihood the way the South did, could exorcise it from their economies without much penalty.
In contrast, slavery becomes a gigantic part of the South. Not just its economy but its society, culture, politics, and values as well. Slavery touches everything about what it means to be a Southerner (black or white). The difference in scale is overwhelming. The nature of slavery in the US is so dominated by its practice in the South and so marginal to the identity or the North that, in general US history courses like those in most high schools and undergraduate programs, it understandably falls by the wayside.
The simple answer to this is that slavery was illegal in every state in the North after 1804, though due to policies of gradual manumission, some slaves still lived in Northern states through the 1840s or later.
The Northwest Ordinance outlawed slavery in the areas northwest of the Ohio River, meaning that new Northern states (those organized after 1804) had antislavery laws or constitutions. Territories south of the Ohio River allowed slavery.
The exception (a state north of the Ohio River that allowed slavery) was Missouri; although Missouri lies on the west bank of the Mississippi River, the Ohio's terminus is well south relative to the rest of the state. Missouri's admission in 1821 was contingent upon the former Massachusetts territory of Maine being admitted as a free state. The Missouri Compromise also allowed slavery in the Arkansas territory (most of modern-day Arkansas and Oklahoma) while prohibiting it in the Great Plains.
There was also native american slaves before african slaves, infact some places infamous for importing slaves, actually started out deporting native american slaves. It wasn't mentioned on this thread just thought i'd let people know. But as we know, the native american population decreased rapidly after contact. So they needed a new source of labor.