After graduating in 1850, he began to read law under Charles Anthony and was admitted to the bar in 1854.

Merriman entered into a law practice with David Noggle, Amos Pritchard, and John M. Berry.

He later moved to Jefferson, Wisconsin, and began a law firm with the state's first lieutenant governor John Edwin Holmes.

[1][2] Just a week before the outbreak of the American Civil War, Merriman was elected the mayor of St. Anthony, Minnesota.

He was re-elected in 1862 but left to serve in the 6th Minnesota Volunteer Infantry Regiment where he had been named captain.