Northumberland Big Sisters amalgamated with Big Brothers Association of Cobourg-Port Hope and District in June of 2004.

The Children's Aid Society of Northumberland applied for and received a Canada Works Grant to sponsor the development of a Big Sisters agency in Northumberland County.  On March 15, 1979, Northumberland Big Sisters became an autonomous organization and was incorporated in August 1979 as a registered charitable, non-profit organization.

Northumberland Big Sisters was proud to be one of the founding members of both the provincial and national umbrella organizations - Big Sisters of Canada and Big Sisters Association of Ontario.  More recently, local members were actively involved in the unification process of bringing these organizations together with Big Brothers and Sisters of Canada.  In July 2001, a new organization know as Big Brothers Big Sisters of Canada was formed.

Big Brothers of Cobourt-Port Hope and District initially began in the early 1970's as a project financed and supported by the Rotary Club of Port Hope and Cobourg area.  The organization was incorporated by Letters Patent on September 20^th 1984.  The first Big Brothers' office was opened at the offices of the Children's Aid Society in 1985 and the agency attained full affiliation status with Big Brothers of Canada in 1986.  Both in 1988 and again in 1995, Big Brothers was named the United Way Agency of the Year.