Although he became a Republican, he opined, while serving as a judge advocate, that President Abraham Lincoln did not possess the constitutional authority to issue the Emancipation Proclamation.

[1] However, when the Civil War began, he argued that the defeat of slavery was imperative for the survival of the nation.

In 1869, he began teaching at Harvard Law School, first as a lecturer, and became a full professor in 1875.

He received honorary Doctor of Laws degrees from Yale University in 1894, and from Harvard in 1895.

[3] Gray wrote two books on future interests, Restraints on the Alienation of Property (1883), and The Rule against Perpetuities (1886).