Was there slavery in Missouri before The Missouri Compromise?

Yes, slavery existed before the Missouri Compromise took place.

Explanation:

The main purpose of the Congress back then was orchestrated as differentiating which states would be free of slavery and which states would not be. Missouri in the year 1891 had opted to be slave state unanimously, which lead to a great unbalance in the union as there would be existence of slave states more than that of the states free from slavery. After the Compromise, Maine was declared a slave free state and thus union gained its balance again.