At its greatest extent, M-146 began at an intersection with US 25 on the south side of Port Huron. It ran north along the west side of the city, on 24th Street, through M-21 and north eastward to the base of the Blue Water Bridge which led into Ontario, Canada. The portion north of M-21 (Lapeer Road) was built to freeway standards.
When the route debuted in 1933 it served as a connector between US 25 and M-21 along the west side of Port Huron. In 1954 a new approach was built from M-21 to the foot of the Blue Water Bridge. M-146 was designated along this route. In 1958, this portion was converted to freeway specifications and in 1964, with the completion of the I-94/US 25 freeway, M-146 was removed from that portion of the route. However, the M-146 designation remained along the stretch of freeway which now serves as the exit 274 interchange on I-94, but was removed from 24th street through town. In 1966, with the completion of a new freeway alignment for M-21 between Wadhams and downtown Port Huron, the M-146 designation is removed from state trunkline system and hasn't been used since.