Founded in 1966 to stop the development of ski slopes on Massachusetts' highest peak, Mount Greylock, CLF has since expanded its advocacy to address both environmental and community issues in all six New England states.

Since that time, CLF's legal advocacy has focused on several natural resources cases, including the cleanup of Lake Champlain, the prevention of overfishing of groundfish – cod, haddock, and flounder – off the coast of New England (resulting in a settlement requiring the National Marine Fisheries Service to produce a management plan to eliminate overfishing), and the protection of the Vermont black bear habitat (by obtaining a federal court injunction halting destructive U.S. logging practices in southern Vermont's fragile Lamb Brook wilderness area, marking the first time an environmental group in the Northeast successfully challenges the U.S. Forest Service's clear-cutting policies).

In a pre-suit settlement with CLF, state highway officials in Massachusetts agreed to implement measures to reduce air pollution, including rail and transit improvements, as part of Boston’s Central Artery project (also known as the Big Dig).

More recently, CLF has continued its work to prevent lead poisoning in children by advocating for bills in New Hampshire and Vermont[4] that require testing of school drinking water sources.

Later, in 2003, CLF claimed victory when the Massachusetts Department of Environmental Protection finalized a schedule requiring the Salem Harbor and Brayton Point coal-fired power plants to significantly reduce harmful emissions and comply with the "Filthy Five" regulations.

CLF also intervened to help the town of Burrillville, Rhode Island, prevent the construction of a large natural gas/oil-fired power plant – which would have polluted the area and contributed to climate change – from being built in the community.

[7] Exposés by InsideClimate News[8] and the Los Angeles Times[9] have confirmed that oil giant ExxonMobil knew as early as the late 1970s that climate change caused by human activities would be devastating if left unchecked.

Its investigation revealed that, despite knowing the harm climate change could cause, ExxonMobil left its oil storage facilities in Everett, Massachusetts, and elsewhere vulnerable to flooding from storms and rising seas.

CLF launched the United States’ first legal action against Exxon for its climate deceit and for Clean Water Act violations at its oil storage facility in Everett, which sits on the Mystic River.