Mills issued a significant ruling in the 1820 case of Rankin v. Lydia which dealt with the rights of slaves brought to the Northwest Territory, where slavery was illegal.
Mills' opinion in Rankin established a precedent that was cited in U.S. courts until the abolition of slavery following the Civil War.
[3] His nephew Joseph Trotter Mills, Wisconsin State Assemblyman and jurist, lived and studied with his uncle.
[5] In 1820, Mills authored the court's opinion in the case of Rankin v. Lydia[6] concerning the status of slaves born in the Northwest Territory where the Ordinance of 1787 outlawed slavery.
[7] In his opinion, Mills was careful to avoid judgments regarding the institution of slavery itself, but upheld the decision of the Shelby County circuit court that the slave (Lydia) ought to be freed under terms of an indenture she had agreed to in Indiana before being sold to a master in Kentucky.
[7] Mills' opinion set a precedent that was adhered to in Kentucky for the next forty-five years and was frequently cited in both northern and southern courts.
In response to the Panic of 1819, the Kentucky General Assembly had passed a law of replevin that was extremely favorable to debtors.
In October 1823, Mills and fellow justices John Boyle and William Owsley upheld the lower courts' rulings.