The first usage of the M-66 designation dates back to around July 1, 1919 with the rest of the original state highway system.

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.

[6] As it leaves Sturgis to the north it crosses a branch of the Michigan Southern Railroad,[7] and it becomes a two-lane surface highway along Nottawa Street.

[6][8] Running through woodland terrain in southern Calhoun County, M-66 passes through Athens, along Graham Lake and continues to the outskirts of Battle Creek.

The highway widens first to a four-lane, limited access expressway south of the Lakeview Square Mall before becoming a full freeway at the interchange with I-94.

The Michigan Department of Transportation (MDOT) calculates the AADT value as a tally of the average number of vehicles using a given stretch of roadway.

[13] The freeway continues 3.374 miles (5.430 km) north into downtown Battle Creek along part of the Kalamazoo River and crossing a branch of the Canadian National Railway and Norfolk Southern Railway[7] before ending at the at-grade intersection with Hamblin Avenue.

M-43/M-66 meets M-50 at a four-way intersection southwest of Lake Odessa near Woodbury, and M-43 turns east leaving M-66 in favor of a concurrency with M-50.

On the south edge of town, the highway passes the county airport and curves to the northeast becoming Dexter Street.

The trunkline runs through the northern part of the county and meets M-44's eastern terminus near Woodard Lake.

The highway jogs west along Main Street in Stanton before returning to a northerly course on a discontinuous section of Sheridan Road.

This area of rural Mecosta County is more heavily forested with rolling hills and sporadic farms.

M-66 continues north passing Merrill Lake before crossing into rural eastern Osceola County at Mesceola Road.

[6][8] The highway continues north and crosses the Great Lakes Central Railroad for the first time in Marion,[7] before entering Missaukee County.

M-66 runs through rolling hills in woodlands through the unincorporated farming community of Lodi north to an intersection with M-72.

The two highways travel west together over the Great Lakes Central Railroad before turning north and merging with US 131 on a route parallel to the rail line.

[15] The highway meanders through more forest lands through the community of Green River to East Jordan.

[6][8] MDOT provides a number of different services to motorists traveling along the state trunkline highway system.

[23] 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.

[34] M-66 was extended southerly from Assyria through Battle Creek to the Indiana state line replacing sections of M-78 in 1965.

The final section of M-78's roadway given to M-66 extended it all the way to the state line, resulting in a north–south trans-peninsular highway from Lake Michigan near Charlevoix to Indiana.

The official explanation was to tie into the history of Native Americans in the area, but the route was also "straight as an arrow".

[9] The backers also promoted the highway as a direct and scenic route to vacation country in the northern Lower Peninsula, avoiding most of the larger cities in the area.

M-66 was given the Green Arrow Route name in Public Act 170 of 1959 between the Indiana state line and Kalkaska.

Public Act 93 of 1976 named all of M-66 in Calhoun County, including the segment that runs concurrently with I-194, as the Sojourner Truth Memorial Highway.

[9] The Cereal City Development Corporation (CCDC) asked the Legislature to amend the memorial designation in 1993.

[9] The Legislature passed Public Act 208 of 1993 to affect the change, restoring "the link between Sojourner Truth and the City of Battle Creek, which was once the center of abolitionist sentiment in the state.