The minimum wage in St. Louis falls by $2.30 an hour Monday, making it a rare city to buck the national trend of municipal pay floors rising above federal and state levels.

Many low-wage workers in the Gateway City will lose raises they received in May, when the minimum wage increased to $10 an hour. A state law taking effect Monday mandates that Missouri municipalities follow the state minimum of $7.70 an hour, nullifying the higher wage St. Louis officials had sought since 2015.

Some...