Northumberland County is situated on the north shore of Lake Ontario, in central Ontario, Canada. It is located east of the Regional Municipality of Durham (formerly Durham County), west of Hastings County, southeast of Kawartha Lakes and south of Peterborough County. The county seat is Cobourg. Together with Durham County, it formed the Newcastle District from 1802 to 1849 and the United Counties of Northumberland and Durham from 1850 to 1973. Effective January 1, 1974, part of Durham County was merged with Ontario County to create the Regional Municipality of Durham. At that time, Northumberland reverted to a standalone county.