He then studied law, was admitted to the bar in Virginia and maintained a successful private practice in Fauquier, Loudoun, and Rappahannock counties.

[2] On August 15, 1835, President Andrew Jackson appointed Horner to be Secretary (and acting Governor) of the Michigan Territory, replacing the popular Stevens T. Mason.

Michigan had satisfied all the requirements set out in the Northwest Ordinance to become a state, however the U.S. Congress had repeatedly rejected or ignored Michigan's petition for statehood, due to two very contentious issues: Jackson, facing reelection in 1836,[citation needed] did not want to alienate Ohio, with its many electoral votes.

So Horner had to appease irate Ohioans as well as deal with an unauthorized, but popular local government that undermined his own authority as Territorial Governor.

Horner was at least partially successful, in that he helped to avert violence (aside from some minor scuffles) and persuaded both parties to wait for the upcoming session of Congress to propose a resolution.

Congress organized the Wisconsin Territory on July 3, 1836, and Horner assumed the office of Secretary, leaving the de facto, if unrecognized, government of the state of Michigan to Mason.