Following 2009–2010 Iranian election protests, Karroubi was put under house arrest in February 2011 – reportedly ordered by the Supreme Leader of Iran – without officially being charged, although he is accused of being a "seditionist" and "traitor". As of 2018, he is still confined to his house.
He has been described as a "moderate" with a "mostly rural" base of support. Karroubi considers himself a pragmatic reformist and now is one of the leaders of the Iranian Green Movement.
He is a founding member and former secretary-general of the Association of Combatant Clerics party. Karroubi is a critic of the Guardian Council and Iran's Judicial System. By appointment of the Supreme Leader, he was a member of the Expediency Discernment Council and an adviser, posts he held until resigning from all his posts on 15 June 2005 after the first round of the 2005 presidential election.
Karroubi studied theology and Islamic studies at seminaries in Qom and Tehran. He studied under notable figures such as Hossein-Ali Montazeri and Ruhollah Khomeini. Karroubi was promoted to Mujtahid on the recommendation of the Grand Ayatollah Yousef Sanei and others. He also studied theology and law at Tehran University. In 1962, he became a lawyer in economy, dealing with the investments of prominent businessmen in Iran.