The Mason/Union Branch was established in February 1973 in the historic Mason Township Hall.

The building, listed on both the Michigan Register of Historic Sites and National Register of Historic Sites and National Register of Historic Places, is the former District No. 5 School.  Described as "the best rural schoolhouse in the state," District No. 5 accommodated 110 pupils in two classrooms.  It was constructed in 1874-1875 at a cost of $3,000.00.  Classes were held here until 1959 when the district was made part of the Constantine school system.

In 1964, the school was transformed into the Mason Township Hall.  The building under went restoration in the late 1980's, thanks to a bequest from Myrtle Hatch.  During that time, the library branch temporarily moved to Baldwin Prairie School.  Restoration was completed in February 1989.