Solon Otis Thacher was born at Hornellsville, New York, on August 31, 1830. The Thacher family was of old New England stock. Solon was the son of Otis and Hannah Kennedy Graves Thacher. Thacher was educated in the common schools of his native town before graduating from Union College. After college he attended Albany Law School and was admitted to practice by the New York Supreme Court in 1856. He entered politics that year and was elected to the New York legislature, serving as a member in the winter of 1857. That same year he opened a law office in Chicago, and in July 1858, he came to Kansas.

Thacher became interested in the Lawrence Republican. In 1859 he was elected as a delegate to the Wyandotte constitutional convention. On the admission of Kansas to the union he was elected judge of the fourth judicial district. In 1864 Thahcer opposed Lane as candidate for governor. He was a member of the state senate in 1881-1883. In 1884 President Arthur appointed him a member of a commission of three to negotiate treaties in South American countries. He was president of the Kansas State Historical Society when he died on August 11, 1895.