In 1870 after three years New York law office, hewas admitted to the bar. He was offered the position of clerk to Maj. David T. Corbin, US attorney at Charleston, S. C., and state senator in the Reconstruction legislature.

Returning to New York , he served for a year and a half (1874-75) as assistant United States attorney for the southern district, quitting the work in disgust at the demands of Republican political bosses and forming a law partnership

He was for two years, an editorial writer for the new morning penny paper,Truth, which soon attained the fourth largest circulation in New York, and which, through his advocacy, was chiefly responsible for the first observance of Labor Day (1882).

Post published in Truth a hasty criticism of the writings of Henry George, which ended in a fast friendship between the two; he became a leading protagonist of the Georgist "Single Tax" philosophy, and this was really his distinctive service for the rest of his life.

In 1913 he became assistant secretary of labor, continuing in office through President Woodrow Wilson's two administrations. He performed his uncongenial work with personal and official fortitude. Impeachment proceedings, urged against him in 1920 because he sought to temper deportation of "radical" aliens with humanity and liberalism, collapsed when he made a brilliant defense, which shamed his inquisitors.

His official position was unsatisfactory to him -- he found himself offending both liberals and conservatives. After his retirement, he continued to live in Washington, giving himself, despite declining health, to industrious writing, much of it in recapitulation of his long and varied life experience. Typical of his more theoretical writings are The Ethics of Democracy (1903), Ethical Principles of Marriage and Divorce (1906), What Is the Single Tax (1926), and The Basic Facts of Economics (1927).

Post left with his widow a manuscript autobiography, Living a Long Life Over Again. His Deportations Delirium Nineteen Twenty (1923) is largely autobiographical and his Prophet of San Francisco (1930) details his connection with Henry George