Al-Mahdi, 45, a journalist, filmmaker, and playwright, was shot in his Baghdad home by assailants using pistols outfitted with silencers.
Al-Mahdi had hosted a thrice-weekly radio show, "To Whomever Listens," on independent Radio Demozy. The show covered social and political issues, including government corruption, bribery, and sectarianism. A Shiite, al-Mahdi had defended the rights of Sunnis.
On his Facebook page, al-Mahdi organized regular pro-democracy demonstrations and publicized threats he said he had received. Growing fearful for his safety, al-Mahdi had stepped down from the radio show about two months prior to his death, news reports said.
Al-Mahdi had spent 18 years in exile, returning to Iraq in 2008. He is survived by his wife and three children.