David Buckel, 60, was pronounced dead after he self-immolated at Brooklyn's Prospect Park early on Saturday, the New York Times reported.
In a suicide note emailed to the newspaper and several other media outlets, Buckel wrote that he had used fossil fuel to symbolise what was happening to the planet.
"Pollution ravages our planet, oozing inhabitability via air, soil, water and weather," the note said.
"Most humans on the planet now breathe air made unhealthy by fossil fuels, and many die early deaths as a result -- my early death by fossil fuel reflects what we are doing to ourselves."
Buckel had long fought for gay rights through the courts and was the marriage project director at Lambda Legal, which was at the forefront of the battle for gay marriage in the US.
"The news of David's death is heartbreaking. This is a tremendous loss for our Lambda Legal family, but also for the entire movement for social justice," Camilla Taylor, the group's director of constitutional litigation, said in a statement.
"David was a brilliant legal visionary. David helped create Lambda Legal's focus on LGBT youth," she said.
The New York Times quoted friends of Buckel as saying that he became involved in environmental causes after leaving Lambda Legal.
His suicide note expressed the hope that his death would serve as a call to action.
"Honorable purpose in life invites honorable purpose in death," Buckel wrote.
Readers seeking support and information about suicide prevention can contact Lifeline on 13 11 14
