"My name is David Buckel and I just killed myself by fire as a protest suicide", the Brooklyn resident wrote.
He had set himself on fire, the note said, in a protest against environmental destruction.
The New York Times said it received an emailed copy of the note, which also 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 wrote that pollution was ravaging the planet and hoped his death would serve as a symbol.
Buckel was the lead attorney in Brandon vs. County of Richardson - a cause that concluded that a Nebraska county sheriff was liable for failing to protect a transgender man, Brandon Teena, who was murdered in Falls City.
A suicide note indicated Buckel used "fossil fuel" in the gruesome suicide as a metaphor for the self-destruction of the planet.
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. She said he "was all about justice, but he was also all about what it means to be human".
Adding, 'Our hearts go out to all who knew him.
In another case he led, the New Jersey Supreme Court ruled that same-sex couples and their children were harmed because they were prevented from having rights granted from marriage. David was a attractive human being who was universally kind to everyone at Lambda Legal, committed to his clients, and devoted to our work'.
Camilla Taylor, who is the current director of Lambda Legal, told the HuffPost that they will honor his life by "continuing to fight for equality".
Mr Buckel was well known for his legal work on behalf of gay, lesbian and transgender people and later worked with several environmental groups.
He was also a well-known ecological activist, working with the NYC Compost Project, funded by the city Sanitation Department and based at the Brooklyn Botanic Garden.
The attorney highlighted the difficulty of improving the world, even for those who tried hard to do so.
In the note, Buckel referenced the Tibetan monk protesters who set themselves on fire in protest the Chinese occupation in Tibet, arguably an image that has captured resistance and protests more powerfully than any other.
'Here is a hope that giving a life might bring some attention to the need for expanded actions, and help others give a voice to our home, and Earth is heard'.
Story by Cleve R. Wootson Jr.