The typed letter, to British Labour Party leader Clement Attlee -- before he became Prime Minister -- sold for $910,880 (£605,000) at a Sotheby's Auction in London Tuesday -- four times pre-sale estimates.
The letter, also signed by Chinese General Zhu De, appeals for support from Attlee's Labour Party in 1937, soon after the Japanese invasion of China.
"We believe that the British people, when they know the truth about Japanese aggression in China, will rise in support of the Chinese people," it said.
In the sale catalog note
, Sotheby's said the "highly important" letter was an early instance of "Mao engaging in international diplomacy and is an exceptionally rare example of Mao's signature."
It's only the second letter signed by Mao to be sold at international auction in recent decades, Sotheby's said.
The letter was written at the encouragement of New Zealand journalist James Munro Bertram, who traveled to Yan'an in China's Shaanxi province. Sotheby's says Bertram presumably translated and typed the letter from the Chinese original, and gave his reasons for doing so in a message to Attlee.
"I asked these people if they would send some message to the British people, and especially to the British Labour Party, of which I hope I may still consider myself a member."
"If I find myself performing the friendly action of an intermediary, this is purely accidental, I was only too glad to do it, because I believe so firmly that international cooperation, and what still survives of the principle of 'collective security', is at the last gasp of its latest hour," he said.
Attlee was in opposition at the time of the letter's delivery but later, in 1945, became British Prime Minister and was in power when Mao proclaimed the foundation of the People's Republic of China in 1949. One year later, under Attlee, the United Kingdom became the first Western country to recognize the PRC.
According to Sotheby's the two men had a three-hour conversation over tea in 1954 -- 17 years after the letter was sent.