Born in New Jersey, Post was educated in New York City. After working at a variety of jobs, including three years at a New York law office, he was admitted to the bar in 1870. He moved to South Carolina to take a position as a clerk in the office of a U.S. attorney; Post was then elected to the state's Reconstruction legislature. He assisted in deposing witnesses for KKK trials in the 1870s. In 1871, he married Anna Johnson. In 1874, he returned to New York and served as assistant U.S. attorney. He quit in 1875 in disgust over the demands of Republican political bosses, and entered a private law partnership. He ended his law practice in 1880s, and became editor and publisher of a variety of newspapers in New York, Cleveland, and Chicago. During the 1880s, he was also active in campaigning for the "Single Tax" movement. His wife died in 1891, and in 1893 he remarried to Alice Thacher, a fellow Swedenborgian. Post was appointed to the Chicago school board ca. 1900, and fought against the embezzlement of school funds, restrictions on academic freedom, and for the right of teachers to organize. In 1913, President Wilson appointed Post as Assistant Secretary of Labor. Post objected to the 1918 Anarchist Exclusion Act, and in 1920, he was the subject of impeachment proceedings after clashing with J. Edgar Hoover and A. Mitchell Palmer over immigrant rights and the deportation of "radical" aliens. He was Acting Secretary of Labor from 1920 to 1927, and died in Washington after a short illness in 1928.