Prior to his Hamas career, he had been a surgeon[4] and worked in Palestinian cities such as Khan Yunis, but was dismissed from this position by Israeli authorities for political reasons.

In response to a campaign of suicide bombings by the Ezzedeen al-Qassam Brigades (EQB), on 10 September 2003 an Israeli F-16 dropped a large bomb over his house in the Rimal neighborhood of Gaza, which only managed to slightly wound him, while his eldest son Khaled, and a personal bodyguard were killed,[2] and twenty others wounded including his daughter Rima.

On 15 January 2008, al-Zahar's son Hussam, a member of IQB, was reportedly killed in an IDF air strike[5] in a car full of Hamas fighters in northern Gaza.

In 2010, al-Zahar revealed to the press that Yasser Arafat had instructed Hamas to launch militant attacks—including suicide bombings—against Israel in 2000, due to peace talks not going anywhere.

"[14] Basim Naim, the minister of health in the Hamas government in Gaza, said Zahar's statements had been misquoted and mistranslated, and that what he did was to "warn that by carrying out these barbaric massacres of children and women, and by destroying our mosques, the Zionists are creating the conditions for people to believe it is justified or legitimate to take revenge....Dr Zahar did not even mention 'Jews' in his comments".

[15] In an interview on New York's WABC radio, al-Zahar was asked by Aaron Klein to comment on the construction of the mosque Park51 near the World Trade Center site.

His other son, a member of Hamas's military wing, the Izz ad-Din al-Qassam Brigades, was killed by Israeli fire in Gaza on 15 January 2008.