As a Senator from New York, Van Buren had voted against the admission of Missouri as a slave state. Following his term as President he was nominated as the presidential candidate of the Free Soil Party, an anti-slavery political party, in 1848. But unfortunately his actions in the field of slavery before and after his presidency didn't match his actions as President. Van Buren considered slavery morally wrong but sanctioned by the Constitution. When the issue of abolishing slavery in the District of Columbia arose during his term, Van Buren made it clear that he was against its abolition. He said as much in his Inaugural Address in 1836. He believed slavery would end by attrition saying:
"I must go into the Presidential chair the inflexible and uncompromising opponent of every attempt on the part of Congress to abolish slavery in the District of Columbia against the wishes of the slaveholding States, and also with a determination equally decided to resist the slightest interference with it in the States where it exists."
Slavery continued in the District of Columbia until April 18, 1862 when Abraham Lincoln abolished it.
Van Buren was President at the time when a case called the Amistad occurred. The Amistad was a Spanish ship carrying Africans captured for slavery, who revolted in an effort to secure their freedom. In the litigation over what to do with the enslaved persons on board, Van Buren sided with the Spanish Government to return the kidnapped slaves. The advocacy of John Quincy Adams in the Supreme Court of the United States led to a different outcome.
Van Buren continued Jackson's policies which resulting in the "Trail of Tears", the expulsion of the Cherokee, Choctaw, Creek, Chickasaw and Seminole nations from Georgia, Tennessee, Alabama, and South Carolina to the Oklahoma territory. To help secure Florida, Van Buren also pursued the Second Seminole War, which had begun while Jackson was in office. The war, which would prove the costliest of the Indian Wars, was highly unpopular in the free states, where it was seen as an attempt to expand slave territory. Fighting was not resolved until 1842, after Van Buren had left office.
With this record of civil rights, it's unclear why, in 1848 the Free Soil Party, an anti-slavery party, nominated Van Buren as its Presidential Candidate. Van Buren, received 291,616 votes against Zachary Taylor of the Whigs and Lewis Cass of the Democrats, but received no electoral votes. The Party's "spoiler" effect in 1848 may have put Zachary Taylor into office in a narrowly-contested election.