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.
In 1981, Hadi's brother Taqi formed the Islamic Front for the Liberation of Bahrain, and appointed Hadi as leader of the organization. The professed aim of the Front was the ‘uprising of all Muslims under Imam Khomeini’. It came to international prominence as the front organization for the 1981 failed coup in Bahrain, which attempted to install Hadi Almodarresi as the spiritual leader of a theocratic Shia state. Almodarresi in addition to heading the IFLB served as Khomeini’s “personal representative” in Bahrain.
As a founding member of the Supreme Council for the Islamic Revolution in Iraq (SCIRI) and senior director of the Islamic Action Organization, Almodarresi was one of the most active figures of the Iraqi opposition in exile. While continuing his religious activities and writing over 250 published books, Almodarresi was closely involved in efforts to expose and bring down the regime in Baghdad. He was also able to escape a number of assassination attempts abroad, including one in Brazil in 1991 as well as two more attempts against his life in Syria by Ba'athist[clarification needed] intelligence operatives in 2001.