The Engineer's Department is in charge of construction and maintenance for all of the Secondary Road System of Marshall County. This includes all roads not located within the city limits of any town, or highways on the Primary Road System which include US 30, IA 14, IA 96, IA 146 and IA 330. As such, Marshall County's Secondary Road Department is responsible for 937 total miles of which 192 miles are paved and the remaining being either granular or dirt surfaced.

The Secondary Road Department is divided into two divisions, Engineering and Maintenance. The County Engineer, who is appointed by the Board of Supervisors, is in charge of both divisions. The Code of Iowa requires the County Engineer to be a Registered Professional Engineer licensed in Iowa.