He rose through the Hamas ranks as a fierce advocate of armed struggle against Israel, imposing his authority among prisoners whilst in Israeli jails.
Slightly built with white hair and a salt-and-pepper beard, the Hebrew speaker knows Israel well, having spent 23 years in jail there with four life sentences for "terrorist activities".
The 55-year-old's election this month makes him the party's second most important figure, behind exiled leader Khaled Meshaal, and observers say it reflects a strengthening of Hamas hardliners.
Israeli commentators say Sinwar's appointment could trigger a countdown to the fourth conflict in 10 years between Israel and the Islamists.
Born in the Khan Yunis refugee camp in the southern Gaza Strip, Sinwar began his political education at a young age, joining Hamas's predecessor, the Muslim Brotherhood-linked Al-Mujama' Al-Islami.
When its founder Sheikh Ahmad Yassin set up Hamas in 1987, Sinwar joined that too.
He also set up the Organisation of Jihad and Preaching, a Hamas intelligence unit that flushes out and punishes "collaborators" -- Palestinians alleged to have provided information to Israel.
Israeli media said Sinwar took a direct and often brutal role in executing alleged agents and commentators have labelled him an ideological extremist who rejects any compromise with Israel.
As recently as 2016 he was involved in the execution of a Hamas military commander allegedly seen as too critical of him, according to a report by a body linked to Israel's Defence Ministry.
Sinwar also established long-running relations with Iran and other countries supporting what Hamas calls "resistance" to Israel.
Arrested by Israel in the late 1980s, he was tasked by Hamas to oversee intelligence gathering and security among prisoners.
"He is a man of security par excellence," said Abu Abdallah, a Hamas official who spent several years in prison with him.
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Like many Palestinians, Sinwar learned Hebrew in jail.
He "knows Israeli culture and society in depth," said Abu al-Abed, a Hamas official.
In 2011 Sinwar was released along with more than 1,000 other Palestinians in exchange for Gilad Shalit, an Israeli solder Hamas had detained for five years.
On his release, Sinwar married.
In 2015 he was placed on the US list of international terrorists. Today he refuses to give media interviews and his movements are a closely-guarded secret.
Over the years, Israel has killed several of Sinwar's mentors. It assassinated Yassin in Gaza in March 2004 and his immediate successor Abdel Aziz Rantissi a month later.
Salah Shehadeh, founder of the armed wing of Hamas in which Sinwar played a key role, was also killed.
Sinwar, who dresses in plain clothes, made a few public appearances after his 2011 release, then disappeared into hiding.
But Abu Abdullah described him as a "charismatic leader."
"He makes decisions in the utmost calm, but is intractable when it comes to defending the interests of Hamas," he said.
Abu al-Hassan, a leader of Islamic Jihad, the other major Islamist force in Gaza, remembers his former co-detainee as a "wise man" who makes "firm decisions."
Sinwar's outlook contrasts with that of his predecessor.
Leader Meshaal has encouraged efforts by Hamas to present a moderate face to the world despite the movement being listed as a terrorist organisation by the United States, the European Union and Israel.
But Sinwar is much less concerned by international attitudes.
"He predicted in prison that one day, when Hamas would speak, the whole world would listen," recalled Abu Abdallah. "And that's exactly what happened".