During the War of 1812, he served with the Tennessee Mounted Volunteers under General Andrew Jackson and eventually rose to the rank of first lieutenant remaining with Jackson until the capture of Pensacola on November 6, 1814. As Jackson left to prepare the defense of New Orleans, Alexander stayed behind in Florida. He later served with Jackson during the First Seminole War, however he was discharged after becoming too ill for active duty.
Returning to Tennessee, he married Mary Shields and lived in Giles County for several years before moving to the Illinois Territory in 1823. Settling in Paris, Illinois, he became involved in farming and mercantilism before being appointed the town's first postmaster, a position he would hold for the next twenty-five years.
In February 1826, he would become appointed Clerk of the County Commissioner's Court. He would hold this position until September 1827, when he resigned to accept a commission in the Illinois Militia from Governor Edward Coles as a Colonel in the 19th Regiment Illinois Militia. He was eventually appointed an aide-de-camp to Governor John Reynolds in December 1831 and, shortly before trouble began to appear on the frontier, he was required to go with Reynolds to Rock Island early the following year.
During the first weeks of June 1832, he was one of hundreds of volunteers to report at Dixon's Ferry as General Henry Atkinson was organizing a second campaign against Black Hawk. On June 16, Alexander Posey and Milton Alexander were elected by the brigades they helped organize to command, respectively, the First and Second Illinois Militias.