Prior to 1929 there were 59 separate power companies serving Northern New York; each company provided power to its local area. In 1929, all 59 companies joined to together to form Niagara Hudson Power Corp. Following internal reorganization in 1950, it became Niagara Mohawk Power Co. In March of 1999, it reorganized again, forming the holding company Niagara Mohawk Holdings, Inc. with Niagara Mohawk Power as a subsidiary. The reorganization was undertaken to separate utility and non-utility operations. In 1999, Niagara Mohawk Power sold its Huntley and Dunkirk coal stations to NRG.
In January 2002, Niagara Mohawk Holdings and the National Grid Group plc completed a merger. Through the merger, Niagara Mohawk Power became a subsidiary of National Grid USA, itself a subsidiary of the National Grid Group plc. In October 2002, the National Grid Group plc and Lattice Group plc (with Transco as a subsidiary) merged, forming National Grid Transco. National Grid Transco is a publicly held company listed on the New York and London Stock Exchanges.
National Grid was originally owned by 12 regional English electric companies following privatization in 1990
 Lattice was spun off from British Gas in October 2000; British Gas was privatized in 1986.