Sidney Rittenberg, an American from South Carolina, was one of the few non-Chinese people to become a senior member of the Chinese Communist party. Known in China, and still revered, as Li Dunbai (which sounds like Rittenberg to Chinese ears), he was an active participant in the communist revolution of the late 1940s and its aftermath.

An intimate of Mao Zedong, Zhou Enlai, Deng Xiaoping and almost every other veteran revolutionary, Rittenberg, who has died aged 98, gained prominence at the Broadcast Administration in Beijing, one of the most important agencies of government, and for a few months in 1967 was the director of Radio Peking, on which he occasionally broadcast in his distinctive southern US accent.

Rittenberg also translated Mao’s Complete Works and the Little Red Book into English and became a leading rabble-rouser in the Cultural Revolution that began in 1966. At that time he would address rapturous crowds of up to 100,000. Jiang Qing, Mao’s wife and the leader of what would come to be known as the Gang of Four, once commented acidly that, at 45, Rittenberg was a little old to be a Red Guard, but he pressed on regardless, until Jiang Qing, thought to have been jealous of this popular foreigner, had him thrown into jail for a 10-year term.

The convulsions of a China constantly reinventing itself led to Rittenberg twice falling foul of the leadership. Of his 35 years in China, he served a total of 16 imprisoned in solitary confinement, accused of being an American spy. Disillusioned with communism, he returned to the US in 1980 with his wife, Wang Yulin, whom he married in 1956, and their four children.

There, he founded Rittenberg Associates, a consulting company that helped businesses from Colgate Palmolive to Warner Music to Intel, Microsoft and PricewaterhouseCoopers to establish themselves in China.

The son of Muriel (nee Sluth) and Sidney Rittenberg, he was born into a prominent Jewish family in Charleston, where his father was a member of the city council. Sidney Jr dropped out of the University of North Carolina, where he was studying philosophy, aged 19, and became a trade union and civil rights activist. Having joined the US army the day after Pearl Harbor, he eventually arrived in China as an idealistic GI interpreter at the end of the second world war.

The corruption and inequalities of life in China under the Chiang Kai-shek nationalists shocked him, and, honourably discharged, he made contact with the communists in Shanghai and was soon trekking for 45 days across China to join Mao’s guerrilla army at Yan’an.

After the communists won power, he was asked to stay on as, in his words, “an engineer to build a bridge from the Chinese people to the American people”. Mao had always been fascinated by the US and, while camped out at Yan’an, would spend hours sequestered with Rittenberg going through old copies of American magazines and asking questions about the US. Rittenberg also did simultaneous translations of Laurel and Hardy films, which Mao loved.

His first spell in prison, of six years from 1949, of which the first was spent in total sensory deprivation, driving Rittenberg to the edge of insanity, came about after Joseph Stalin wrote to Mao warning him that the American was a spy.

His captors never quite seemed to believe the charge, but Mao supported his continued incarceration to test him. “They did say once, ‘If you’re a real revolutionary, you should be able to stand this test,’” Rittenberg said, “and that was all I needed.” He was offered the chance to go back to the US, but decided to stay. “I was just getting into ever deeper study of his writings and deciding he was a genius,” Rittenberg said.

Although he denied all his life that he spied for the US, even in old age Rittenberg would be asked by retired FBI and CIA chiefs whom he had been reporting to in Washington while under his “deep cover” in China. When he insisted he was not a spy, Rittenberg related, the former spooks would typically tap their noses and say: “You’re still very good.”

“I think China has to face the fact that Mao was one of the worst people in human history,” was Rittenberg’s assessment of Mao in old age. “He was a genius, but his genius got completely out of control, so he was a great historic leader and a great historic criminal. He gave himself the right to conduct social experiments that involved upturning the lives of hundreds of millions of people, when he didn’t know what the outcome might be. And that created famines in which tens of millions died, and a revolution in which nobody knows how many died.” At a personal level, he said: “I didn’t feel any warmth, although he said nice things about me.”

Rittenberg explained that his idealism and the belief that he was taking part in the development of a new and better world blinded him to the atrocious persecution and murder of even close friends of his. “It’s a kind of corruption, exactly the kind of corruption that ruins the whole thing,” he said. “I believed I was part of history. That’s what you get with ideology and power. You learn to harden your heart in the name of the wonderful new world you’re building. Once you do that, you do all kinds of things. I did.”

Rittenberg, in later years at least, had an impish sense of humour, loved jokes – especially Jewish jokes – and was called upon as a commentator on Chinese affairs by both western and Chinese media. In 1993 his memoir, The Man Who Stayed Behind, was published, and in 2012 he was the subject of the documentary The Revolutionary.

He taught into his 90s at several US universities. On his office door at one there was a sign that read “S. Rittenberg. History.” Under it, he wrote: “Not yet.”

He is survived by Yulin and their children, Jenny, Toni, Sunny and Sidney Jr, and four grandchildren.

• Sidney Rittenberg, writer and political activist, born 14 August 1921; died 23 August 2019