Sanilac County is a county located in the Thumb region of the U.S. state of Michigan. As of the 2020 Census, the population was 40,611. The county seat is Sandusky. The county was created on September 10, 1822, and was fully organized on December 31, 1849. Sanilac County is considered to be part of the Thumb of Michigan, a subregion of the Flint/Tri-Cities. Sanilac County enjoys seasonal tourism in towns such as Lexington, Port Sanilac, and Carsonville. Sanilac County is economically attached to St. Clair County and Huron County and is largely composed of nearly flat areas of rich soil.

Tuscola County is a county in the Thumb region of the U.S. state of Michigan. As of the 2020 census, the population was 53,323. The county seat is Caro. The county was created by Michigan Law on April 1, 1840, from land in Sanilac County and attached to Saginaw County for administrative purposes. The Michigan Legislature passed an act on March 2, 1850, that empowered the county residents to organize governmental functions.