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The U.S. state of Michigan first required its residents to register their motor vehicles and display license plates in 1910.
Passenger baseplates 1910 to present
In 1956, the U.S. states and Canadian provinces came to an agreement with the Automobile Manufacturers Association that fixed the size for license plates for vehicles, except those for motorcycles, at six inches in height by twelve inches in width, with standardized mounting holes. The 1955 (dated 1956) issue was the first Michigan license plate that complied with these standards.
References
Vehicle registration plates of Michigan Wikipedia(Text) CC BY-SA