His father, Ayatollah Hajj Seyyed Mostafavi Kashani, was a noted scholar of Islam in his time. Abol-Ghasem was trained in Shia Islam by his religious parents and began study of the Quran soon after learning to read and write.At 16, Abol-Ghasem went to an Islamic seminary to study literature, Arabic language, logic, semantics and speech, as well as the principles of Islamic jurisprudence, or Fiqh. He continued his education at the seminary in Najaf in the Qur’an and Hadiths as interpreted in Sharia law, receiving his jurisprudence degree when he was 25.
His son Mostafa died in an accident in 1955; the newly appointed prime minister, Hossein Ala’, escaped an assassination attempt at the funeral.According to British intelligence, around this time two of his sons were involved in a lucrative business buying and selling import-export licenses for restricted goods.One of Kashani’s children, Mahmoud Kashani, went on to become head of the Iranian delegation to the International Court of Justice in The Hague, Netherlands, in Iran’s case with the United States and a presidential candidate in the Iranian presidential elections of 1985 and elections in 2005. His second son is Ahmad Kashani, a former member of the Iranian parliament.