The first usage of the M-66 designation dates back to around July 1, 1919 with the rest of the original state highway system. At the time, the highway only extended between Lowell and Lakeview, a route now covered by M-91. The highway has been lengthened in a series of extensions north and south starting in 1925. A rerouting in 1944–45 removed M-66 from its original 1919 routing to replace another highway south of Six Lakes, the change that spawned M-91. The last big extension in 1965 resulted in the modern trans-peninsular highway route. The last modifications were shorter reroutings in the 1970s.