Last week Mr Nicklinson, from Melksham, Wiltshire, lost his High Court case to allow doctors to end his life.
Mr Nicklinson's family solicitor said that he had refused food from last week.
Lawyers for another man, 47, who lost his High Court case alongside Mr Nickinson, are to appeal against the decision. Tony Ncklinson Locked In Syndrome,
They said it denied their client "the opportunity to take the necessary steps to end his own life".
Mr Nicklinson's family solicitor Saimo Chahal said he died at home at about 10:00 BST in the presence of his wife, Jane, and two daughters, Lauren and Beth.
She said: "Jane told me that Tony went rapidly downhill over last weekend, having contracted pneumonia."
She added: "Jane said that, after Tony received the draft judgment on 12 August refusing his claim, the fight seemed to go out of him.
"He said that he was heartbroken by the High Court decision that he could not end his life at a time of his choosing with the help of a new doctor.
"He could not understand how the legal argument on his behalf could not succeed."
She said Mr Nicklinson had told her two days after the ruling he was "crestfallen, totally devastated and very frightened".
He had added: "I fear for the future and the misery it is bound to bring.
"I suppose it was wrong of me to invest so much hope and expectation into the judgment but I really believed in the veracity of the argument and quite simply could not understand how anybody could disagree with the logic.
"I guess I forgot the emotional component."
Ms Chahal said Mr Nicklinson had made an advanced directive in 2004 refusing any life-sustaining treatment.
Also Read More: http://www.bbc.co.uk/news/uk-england-19341722
