Mohammad Shah, son of Abbas Mirza, FathAli Shah's prince crown was the third Qajar Shah. When the news of FathAli Shah's death reached Mohammad Mirza in Tabriz, he coronated in the same city, and proceeded with his forces to Tehran and encountered his uncle, Sultan Ali Mirza Zil-los-Sultan's forces in the vicinity of Tehran. Zil-los-sultan's forces were defeated and collapsed. Again, one of his uncles, Hassan Ali Mirza (Farmanfarma) Announced himself Shah of Iran. However, Mohammad Shah's forces managed to destroy his forces in southern Qomsheh.
In his early days of reign, he appointed Qaem Maqam Farahani as his grand vizir. But less than a year after, through intrigues of Sir John Campbell, then the British minister in Iran, he ordered the arrest and the murder of that great man. Yahya Dowlatabadi says:
….On the whole they kept Qaem Maqam for five or six days in a pond room in the basement of Negarestan without any food should he die when his strength diminishes. Eventually, the head executioner, Safar Esmaeel Khan Qarajeh Daghi entered the basement with some executioners and put some handkerchief in his mouth and suffocated him….
Mirza Aboouolqassem Qaem Maqam's murder was one of the shocking crimes of Mohammad Shah's reign. Every body believed in his ability and patriotism, and the Qajar king could make the best of his potentialities.
Subsequent to Qaem Maqam's murder, Hajj Mirza Aghassi was appointed as grand vizir. During his tenure of his office, Mohammad Shah instigated by the Russians, sent a military expedition to Herat. However, due to the strong opposition of the British, they were defeated, and returned to Iran. Military campaign against Herat and plans to dominate Afghanistan were the main events under Mohammad Shah's reign which were followed by many grievous consequences and led to more British influence and intervention in Iranian affairs in the country.
Defeat in battlefield, the British intrigues, cholera and pest epidemics, inefficiency of Hajj Mirza Aghassi created an atmosphere of disorder in Iran. Meanwhile, Mohammad Shah suffered ill health and died in his forty three years of age, having ruled for 14 years in Iran. He had nine children; five sons and four daughters. Among his wives, Mahd-e-Olia enjoyed great power and meddled in the appointment and the removal of high authorities, and exercised influence in the handling of affairs. She had two children: her son, Nasser-ad-din became the shah of Iran after his father's death, and her daughter Malakzadeh Ezzat-od-dowleh got married to Mirza Taqi Khan Amir Kabir. Mohammad Shah's other children were: Abbas Mirza, Abdol-Samad Mirza, Mohammad Taqi Mirza, Ibrhim Mirza, Assieh Khanom, Ozra Khanom, and Zahra Khanom.