he graduated from the West Point in 1853 and after serving as US Army officer in the Seminole War, he left the Army and studied law. He started a law practice in St. Louis, Missouri, served as US District Attorney (1839-43) and as judge of the court of common pleas (1834-49). In 1852, he moved to Maryland, where he practiced Supreme Court law and was a solicitor in the Court of Claims (1855–58). Blair took an active part in the presidential campaign of Abraham Lincoln and after his election to President, Lincoln appointed him to his cabinet as the 20th US Postmaster General in 1861. As Postmaster General, Blair made such reforms and improvements as the establishment of free city delivery, the adoption of a money order system and the use of railway mail cars were instituted for mail delivery. He resigned his post in September 1864 and served as general superintendent of the US Railway Mail Service, until his death.