Cortland is a city in Trumbull County, Ohio, United States located on the eastern shore of Mosquito Creek Reservoir. Walnut Run is the creek that runs through town. The population was 7,104 at the 2010 census. It is part of the Youngstown-Warren-Boardman, OH-PA Metropolitan Statistical Area. Cortland became a village in 1874 with the construction of a railroad depot. It became a city in 1980 when its population exceeded 5,000 people.