M-66 was first signed along a roadway by July 1, 1919 between M-16 (now M-21) at Lowell and M-46 near Lakeview. The designation was extended in 1925 along M-46 to Six Lakes and then north to M-13 (now US 131) in Lodi. A further extension north from Lodi in 1929 or 1930 along M-131 to Mancelona, a short segment of M-88 and north to US 31 in Charlevoix. The southern end was extended to US 16 south of Lowell by 1931. A section of the northern extension was marked on maps through 1933 as "under construction". That section was cancelled in favor of another routing near Mancelona. A short bypass of Six Lakes added about a mile to the length of the roadway in 1936. The M-131 concurrency was switched to a US 131 concurrency when the latter was extended in 1939. This concurrency was shortened just before World War II when the Michigan State Highway Department (MSHD) rerouted US 131 along a new road between South Boardman and Kalkaska.