Ali ar Rida was born in Medina to the seventh Imam, Musa al Kazim and Ummul Baneen Najma. He was born one month after his grandfather, Jafar as Sadiq died. Like his father and grandfather, his education came at the hands of his father.
His father died in 799, when Ali was 35, and he was given the responsibility of the Imamate. Ali was not looked upon favorably by Harun Rashid, and the people of Medina were disallowed from visiting Ali and learning from him. After the death of Harun Rashid, his two sons began fighting for control of the Abbasid empire. One son, Al-Amin, had an Arab mother and thus had the support of Arabs, while his half-brother Al-Ma'mun had a Persian mother and the support of Persia. Al Ma'mun believed that Persia was sympathetic to the Hashemites and asked for Ali to meet him in Persia. Ali left his only son, Muhammad at Taqi and his wife, and set out for Merv.
After defeating his brother, Al Ma'mun named Ali ar Rida his sucessor. He hoped to win Shia support through this move, but the passage of caliphate would only occur if Ali outlived Al Ma'mun. Al Ma'mun even changed the black Abbasid flags to green, the traditional color of the house of Ali. However, Ali wouldn't outlive Al Ma'mun, and he died on May 26, 818 in Iran while accompanying Al Ma'mun at Tus. Some believe that he was poisoned by Al Ma'mun, but whether or not this is true is hard to determine.
Ali ar Rida is buried in Mashhad, and the city grew up around his shrine. It is one of the most important shrines to the Twelver Shia.