During a conservative era of the Supreme Court he became the chief liberal dissenter on the court and for many years, the only dissenter. In his dissenting opinions in the Civil Rights Cases of 1883 he spoke out for the rights of African Americans guaranteed by the 13th, 14th and 15th Amendments. His dissent against the segregation of black people in the infamous Plessy decision of 1896 was a legal landmark, and used much the same reasoning that the Court later followed in the Brown decision of 1954 that ended legal segregation of public schools. He was in the minority in favor of the constitutionality of the federal income tax when it first came before the Supreme Court. And yet, John Marshall Harlan had been a slaveowner, as his father was before him.
History is full of such contradictions.
Justice Harlan had a black half-brother, Robert J. Harlan, whom the family taught to read and write. They allowed him to go into business for himself in Harrodsburgh, Lexington, and Cincinnati. In later life he became a racehorse owner and trainer, a leading local Republican, and later a federal officeholder in Washington. Robert Harlan won't be found in Alpheus Harlan's history, but his life is on record in other histories and documents. Harlans were on both sides of the Civil War, but without having an actual count, I would say more of them were on the Union side.