Horatio Seymour attended the local schools, and was tutored by his brother in law, the Reverend Truman Marsh.

Seymour then moved to Middlebury, Vermont, completed his legal training in the office of Daniel Chipman, and was admitted to the bar in 1800.

He was active in the Episcopal Church, and the management of the Addison County Grammar School and Middlebury College.

His son Horatio Seymour Jr. (1813–1872) was an attorney in Buffalo, New York and served as Erie County's surrogate court judge.

Seymour's daughter Emma was the wife of Phillip Battell and they were the parents of two children before her death in 1841.

His son Henry served in the United States Army during the Second Seminole War, and died in Boston in 1847.

In 1847 Yale University awarded Seymour an honorary LL.D., an event timed to coincide with the fiftieth anniversary of his college graduation.