He tracked down suicide bombers and their handlers from his father’s organisation, the Haaretz newspaper said.
Information supplied by him led to the arrests of some of the most- wanted men by Israeli forces, including Marwan Barghouti, a Fatah leader tipped as a potential president, who was convicted of masterminding terrorist attacks, along with one of Hamas’s top bombmakers, Abdullah Barghouti, who is no relation of the jailed Fatah chief.
Mr Yousef had harsh words for the movement that his father helped to form, and which now rules the Gaza Strip after a bloody takeover in the summer of 2007. “Hamas cannot make peace with the Israelis,” he said. “That is against what their God tells them. It is impossible to make peace with infidels, only a ceasefire, and no one knows that better than I. The Hamas leadership is responsible for the killing of Palestinians, not Israelis.”
Mr Yousef’s former Israeli handler, identified only as Captain Loai, praised the resolve of his agent, whose codename derived from the colour of Islam and Hamas’s banner and from his exalted position within an organisation that regularly kills those suspected of collaborating with the Jewish state.
“So many people owe him their life and don’t even know it,” he said. “The amazing thing is that none of his actions were done for money. He did things he believed in. He wanted to save lives. His grasp of intelligence matters was just as good as ours — the ideas, the insights. One insight of his was worth 1,000 hours of thought by top experts.” ... On one occasion he followed a bomber from Manara Square in the centre of Ramallah, just north of Jerusalem.
“We didn’t know his name or what he looked like — only that he was in his 20s and would be wearing a red shirt,” said the former handler. “We sent the Green Prince to the square and with his acute sense, he located the target within minutes. He saw who picked him up, followed the car and made it possible for us to arrest the suicide bomber and the man who was supposed to give him the belt. So another attack was thwarted, though no one knows about it.
Helping Israeli intelligence foil suicide bomber attacks isn't the only thing Mosab has done which may cause anger back in his homeland. The son of Hamas's founder also converted to Christianity a decade ago, though he kept his conversion a secret until he was able to emigrate to the US.
Mr Yousef said that his questioning of Islam and Hamas began early. His father, a pragmatist who has even suggested Hamas would be willing to talk to Israel under certain conditions, would often accept his concerns, such as the targeting of civilians.
Mr Yousef said that his doubts about Islam and Hamas crystallised when he realised not all Hamas leaders were like his father, a moderate who he describes as "open-minded, very humble and honest".
Mr Yousef said that he was appalled by the brutality of the movement, including the suicide bombers seeking glory through jihad.
"Hamas, they are using civilians' lives, they are using children, they are using the suffering of people every day to achieve their goals. And this is what I hate," he said.
It was after a chance encounter nine years ago with a British missionary that Mr Yousef began exploring Christianity.
He found it "exciting", he said, and began secretly studying the Bible, struck by the central tenet "love your enemies".
He said that after he converted to Christianity, he decided he had to escape and "live my life away from violence because I couldn't coexist with that situation as a Christian."
"I was thinking, what is my responsibility now? To see people dying every day or to stand up and say, this is wrong, this is right and be strong about this? So I had to make this move."
Mr Yousef said that his father, who has spent more than a decade in Israeli jails for his involvement with Hamas, was in prison when he "got the worst news in his life" - that his son had become a Christian and left Ramallah. "But at the same time he sent me a message of love.
"Everybody is asking him to disown me. You understand if he disowns me he will give terrorists a chance to kill me. "He loves me as a son and he believes that what I've done was something I believed in, but at the same time it's very difficult for him to understand and he won't be able to understand."