In Thoreau's time it was legal to own a slave?

While Henry David Thoreau was alive (1817-1862), it was legal to own slaves in at least some parts of the US.

However, Thoreau spent pretty much his entire life in Massachusetts, where slavery was ... um ... well, it's complicated.

Slavery had at one time been legal in Massachusetts, and the 1778 proposed state constitution legally recognized slavery and banned free blacks from voting. However, it was rejected at the polls (not specifically because of the slavery issue, though). The constitution that was eventually passed in 1780 declared "all men are born free and equal, and have [...] the right of enjoying and defending their lives and liberty."

That doesn't actually explicitly say anything about slavery, but it was enough for pro-abolition Massachusetts courts to run with, and in a landmark decision in 1783, Massachusetts Chief Justice William Cushing held that slavery was not compatible with the Massachusetts constitution ...

... except that it left the door open for a person's liberty to be forfeit "by some criminal conduct or given up by personal consent or contract" (basically holding that indentured servitude was permissible). However, this isn't exactly what is usually meant by "slavery".

So: where Thoreau lived, slavery was at least de factonot legal, even if no actual law forbidding it in so many Words was ever passed by the legislature (they seem to have been content to allow the Court's interpretation to rule).