On January 11, 1805, President Thomas Jefferson signed an act establishing the Michigan Territory.
When Ohio became a state in 1803, the present-day state of Michigan became part of the Indiana Territory. Since the territorial capital was in Vincennes — a long distance (350 miles and no roads) from Michigan’s population center of Detroit — Michiganders lobbied for their own territory. Communication between the Indiana territorial leadership and Detroit was so poor, that when Gov. William Henry Harrison ordered an election to be held in the territory, the order never reached Detroit and Detroiters never voted.
The law creating the Michigan Territory took effect in July 1805. It included the Lower Peninsula and the tip of the eastern Peninsula. Over the next several years the territory was expanded to include parts of Wisconsin and Minnesota.
Sources :
Michigan Territory wikipedia entry
The Mitten, a publication of Michigan History magazine.
Michigan is Amazing