On March 17, 1847, Michigan’s capital moved from Detroit to Lansing following legislation signed the day before by Gov. William Greenly. The move out of Detroit was made for at least two reasons.
For one, Detroit’s location in the southeast corner of the state was too hard for all residents of the state to get to before the era of automobiles, airplanes and highways. Lansing was smack dab in the middle of the state.
Secondly, fears still existed about having the capital so close to the Canadian border, it being about 30 years removed from the War of 1812. After the Legislature packed its bags, the old Capitol in Detroit was turned into the city’s first — and only — high school, Capitol Union.
Sources: Historic Detroit.org and Mich-Again’s Day