Almodarresi was born in 1957 to a family with a long line of top-ranking scholars that dominated the hawza (Islamic seminaries) for many years in Karbala, Iraq. His family includes supreme religious jurists (marja’a) such as Grand Ayatollah Mirza Mahdi Al Shirazi (grandfather), Grand Ayatollah Mohammad Shirazi (uncle) and Grand Ayatollah Sadiq Shirazi (uncle), Grand Ayatollah Ali Al-Sabzowari (cousin), Grand Ayatollah Sayed Abdul Hadi Al-Shirazi (great uncle), as well as Grand Ayatollah Mohammad Taqi Al Modarresi (brother).
Almodarresi started his religious education in the Islamic seminary of Karbala at the age of three and actively sought his religious studies under the auspices of many high ranking scholars. He completed the secondary part of the curriculum by the age of nine. Due to his distinguished abilities Almodarresi received the recognition of several maraje’ who appointed him as their special representative. While engaged in preaching Islam in Bahrain and at the young age of 26 Grand Ayatollah Sabzewari and Grand Ayatollah Mar’ashi Najafi also awarded Almodarresi power of representation in which they praised him and labeled him as “scholar worthy of taking a leadership position” and urging Muslims to follow his lead.
Almodarresi's advocacy of political freedom and strong stance against terrorism started from an early age when Saddam Hussein came to power in Iraq. Seventeen members of his wife’s family were executed by Saddam's regime or simply disappeared in the notorious Baathist penitentiaries. Almodarresi wrote the first book openly attacking the Iraqi regime ever to be published by a religious scholar. Published under a pseudonym in Beirut, the book was titled No To Rulers of Iraq and sparked a massive political crisis in Baghdad and caused the Baathist regime to issue an ultimatum for the removal of all Lebanese nationals from Iraq within 72 hours.
Almodarresi eluded execution by moving from house to house, often living in cellars for months and traveling in disguise. His uncle Ayatollah Sayed Hassan Al Shirazi was gunned down by Iraqi government assassins in Beirut for his role as a key opposition figure to Saddam's regime.
With the escalation of the Ba’athist repression, Hadi fled Iraq and found sanctuary in Bahrain, where he gained citizenship in 1974, and rose to international prominence. In 1979, Hadi Almodarresi left Bahrain for Iran, either because his passport expired, or because he was deported.