In March 2000, National Grid Group acquired United States companies New England Electric System and Eastern Utilities Associates.

[12][13] In August 2004, National Grid Transco agreed to sell four of its regional gas distribution networks for a total cash consideration of £5.8 billion.

[15] In February 2006, National Grid announced that it had agreed to buy KeySpan Corporation,[16] a gas distributor and electricity producer in the United States, for $7.3bn (£4.1bn) in cash.

[17] The acquisitions of the two natural gas delivery companies doubled the size of National Grid's American subsidiary, creating the second largest utility in the United States with more than 8 million customers.

[18] In May 2007, National Grid formed a joint venture with the Dutch transmission operator TenneT for a 260-kilometre (160 mi) 1,000 MW BritNed DC link between the Isle of Grain in Kent and Maasvlakte, near Rotterdam.

[26] In the spring of 2011, National Grid sold off its services in New Hampshire, after their request to increase gas and electric rates was denied.

[28] In June 2016, the Energy Select Committee argued that the company faced too many conflicts of interest, particularly with regard to its ownership of international interconnectors.

[32] At the time of its announcement, National Grid was the largest energy company based in the United Kingdom to join the alliance, according to publicly available financial figures of 2018.

[33][34][35][36] Later in the year, the company moved ownership of its operations in the United Kingdom to Luxembourg and Hong Kong, to protect itself from Labour's nationalisation plans.

[37] A spokesman said, "Labour's proposals for state ownership of National Grid would be highly detrimental to millions of ordinary people who either hold shares in the company or through their pension funds."

[50] In the past, National Grid operated the electricity transmission networks in England and Wales and Scotland in its role as the transmission system operator for Great Britain, working to balance supply and demand in real time, as well as coordinating markets and auctions to ensure sufficient future supply, and exploring initiatives such as demand-side response measures to reduce peaks in electricity demand.

[64] Also in 2019, the company spent $100 million to acquire solar and wind generation from Geronimo Energy, in partnership with Washington State Investment Board.

[68] Contract negotiations with Massachusetts gas workers represented by the United Steelworkers broke down in June 2018, and the company locked out more than 1,000 employees, cutting off healthcare and pay.

Cuomo threatened "to revoke the company’s authority to operate its gas franchise in New York City and Long Island, for failing to provide customers with reliable service," according to Utility Dive.

Local activists have pushed back on the project questioning its necessity, safety, additional cost to customers and noting how it will work against the recently enacted New York State Climate Leadership and Community Protection Act.

[72] In the years 2012 to 2020, National Grid was accused of manipulating energy efficiency programs and overcharging residents of Rhode Island by approximately US$2.2 million.