Polly Strong (c. 1796–unknown) was an enslaved woman in the Northwest Territory, in present-day Indiana. She was born after the Northwest Ordinance prohibited slavery. Slavery was prohibited by the Constitution of Indiana in 1816. Two years later, Strong's mother Jenny and attorney Moses Tabbs asked for a writ of habeas corpus for Polly and her brother James in 1818. Judge Thomas H. Blake produced indentures, Polly for 12 more years and James for four more years of servitude. The case was dismissed in 1819.

In 1819, attorneys John W. Osborn and Amory Kinney, sought to test the legality of slave arrangements made prior to 1816. They sued for Strong's freedom at the Knox County Circuit Court in Polly v. Lasselle in 1820, but the court ruled that she was to remain enslaved. The case was appealed to the Indiana Supreme Court in State v. Lasselle. Based upon the 1816 Constitution of Indiana, the justices ruled that "slavery can have no existence" in Indiana. Strong was freed. Other enslaved people were not automatically freed, but there was a precedent that others could use in the courts. In 1821, the Mary Clark v. G.W. Johnston case was tried in the Indiana Supreme Court, which ruled that indentured servitude was no longer legal in Indiana. The number of enslaved people reduced over the next couple of decades. There were three slaves in the state in both the 1830 and 1840 censuses.

By 1746, African and Native American enslaved people were owned by Catholic priests and French traders, and the practice continued during the period when Great Britain controlled the area that is Indiana. The frontier north and west of the Ohio River became the Northwest Territory in 1787. Article VI of the Northwest Ordinance prohibited slavery and involuntary servitude. Many slaveholders took this to mean that the ordinance excluded people who already owned slaves.

In 1800, the Indiana Territory was formed. Many slaveholders immigrated to the territory bringing their slaves with them. The territory passed legislation that allowed for the slaves to be indentured within the territory. This law allowed slaveholders to continue in their ownership of slaves that were purchased outside of the state, even if the slaves came to reside in Indiana.

Slavery and involuntary servitude were prohibited by the 1816 Indiana Constitution. This ban brought the previous laws that had permitted slavery into conflict, and the issues of whether preexisting slave arrangements were still legal was brought before the courts on several occasions. Each time the circuit courts had denied or approved the freedom of the slaves on an individual basis. Black people continued to remain enslaved or indentured servants after 1816. There were 190 slaves in Indiana in 1820, 118 of them in Knox County, Indiana.

In his article "Almost a Free State", historian Paul Finkelman enumerated three ways for enslaved people in Indiana to become free. They could run away, they could hire attorneys to file freedom suits in the court, or their owners could decide to give them their freedom.

Around 1796, Polly Strong was born in the Northwest Territory to an enslaved woman, Jenny, who was owned by Antoine Lasselle who lived near Fort Wayne. Her father was someone with the surname of Strong. Prior to Strong's birth, Jenny, had been kidnapped by Native Americans. She was sold to Antoine Lasselle after the Treaty of Greenville (1795). Strong's brother, James, was born around 1800. They were separated when Strong was sold to Joseph Barron. James was later sold to someone with the surname of LaPlante. At around the age of ten, Strong was purchased by Hyacinthe Lasselle, an innkeeper and trader from Vincennes, in what was then Indiana Territory (1800–1816). Lasselle later purchased James.

Strong was baptized on April 11, 1819, with the name Marguerite at St. Francis Xavier Catholic Church in Vincennes. At the time, she was identified as a mixed-race woman. She may not have been treated well by Lasselle. Strong was described as pretty with a "bright disposition" and "pleasing manners".