He was the only son of Imam Ali Reza (a.s.) The Prophet (S.A.W.) had said:
"My father be sacrificed on the mother of the 9th Imam who will be a pure and pious Nubian."
Imam Musa Kadhim (A.S.) had told one of his companions that his daughter-in-law (Sabika) would be one of the most pious of women and to give her his salaams. She came from the same tribe as the Prophet's wife - Mary Copt who bore the Prophet his son Ibrahim (who died in infancy).

Childhood
The 9th Imam was 5 years old when his father was called to Baghdad by Mamun Rashid to be his 'heir apparent'. When Imam Ali Reza (a.s.) was leaving he saw his son putting sand in his hair. He asked why and the young Imam replied that this was what an orphan did.

His titles, life and works
He became an Imam at the age of 9. Mamun Rashid thought that as all the rulers before him had oppressed the Imams and their schemes had backfired that he would try to bribe the Imams. He tried to make the 8th Imam his heir apparent and give him power and wealth but that also backfired.
He now tried to use power and wealth with the 9th Imam again but from a much younger age thinking that he would be able to influence him. His main purpose was also to make sure that the 12th Imam (whom he knew would bring justice to the world) would be from his progeny and therefore intended to give his daughter Ummul Fadhl to the Imam for a wife. Mamun still continued oppressing the family and followers of the Ahlul-bayt (a.s.).
Mamun called the young Imam (a.s.) to Baghdad from Madinah and offered his daughter. This infuriated his family (Banu Abbas). To prove to them the excellence of Imam even at a young age he arranged a meeting between Imam and the most learned of men at that time - Yahya bin Athkam.
It was a grand occasion with some 900 other scholars present. Imam (a.s.) was first asked by Yahya:
What is the compensation (kaffara) for a person in Ehraam who hunted and killed his prey?"
Imam replied that there were many more details required before he could answer the question:
Did the Muhrim (one in Ehraam) hunt in the haram or outside?
Did the Muhrim know Sharia or not?
Did he hunt intentionally or not?
Did he hunt for the first time or was this one of many times?
Was he free or a slave?
Was his prey a bird or an animal?
Was it big or small?
Had he hunted by day or by night?
Was he baligh or not?
Was he repentant or not?
Was his ehraam for Hajj or Umra?
Yahya was stunned. He looked down and started sweating.
Mamun asked the young Imam to answer the question, which he did, and then Imam asked Yayha a question which he could not answer. The Banu Abbas admitted defeat and Mamun took the opportunity to offer his daughter in marraige to Imam. Imam (a.s.) read his own Nika (the khutba of which is used today) with the Mehr of 500 dirhams. Imam wrote a letter to Mamun that he would also give Ummul Fadhl Mehr from the wealth of Aakhira. This was in the form of 10 duas which were for fulfilling any hajaat (desires) [Chain of narrators upto Prophet - Jibrail - Allah]* . Thus his title Al-Jawad (the generous one).
* These duas are found in Mafatihul Jinaan (pg 447 - In margin)
Imam lived for a year in Baghdad with Ummul Fadhl. She was very disobedient to Imam. When she found out that Imam had another wife (from the progeny of Ammar-e-Yasir) and that there were also children, she was jealous and angry realizing that her father's plan had failed. She complained to her father who also realized that his plan, to keep the 12th Imam in his progeny, had failed. He was enraged and in his rage he drank heavily and went to the 9th Imam's house and attacked Imam with a sword. Both Ummul Fadhl and a servant saw the attack and believed Imam was dead. Mamun on waking next morning realized the consequences of his attack and was thinking of arranging the disposal of Imam's body when he saw Imam well without a scratch on him. He was confused and asked Imam who showed him an amulet which is called Hirze Jawad. Imam told him it was from his grand mother Bibi Fatima Zahra (a.s.) and kept the wearer safe from all except the angel of death. Mamun asked Imam for it and Imam gave him one.
Now Mamun was scared and tried a new tactic. He tried to deviate Imam by sending him beautiful girls and musicians. When he realised nothing was working he let Imam return to Madinah.
Imam used this time to prepare the masails of Taqleed and Ijtihaad in preparation for the 12th Imam knowing that both the 10th and the 11th Imam would spend most of their lives in prison. He also prepared the people of Madinah teaching true Islam knowing that this would be the last time they would be able to receive guidance directly from an Imam for a long time.
Ummul Fadhl continuously complained of Imam to her father who sent her letters back.
Mamun died in 218 A.H. and was succeeded by his brother Mo'tasam Billah. He openly announced that all Shias were not Muslims. He said it was required for people to kill and prosecute Shias, and to destroy property belonging to Shias.
Ummul Fadhl now started complaining to her uncle who was sympathetic to her. Mo'tasam called Imam to Baghdad. He asked Imam to pass judgment of how to punish a thief. Imam said only fingers could be cut as the palms were for Allah (as in Qur'an - it is one of the wajib parts to touch the ground during sajda). As this decision was contrary to the decision of the other 'Ulama' it strengthened the position of the Shias. The other 'Ulama' complained to Mo'tasam.

Martyrdom and Burial
With instigation from both the 'Ulama' and Ummul Fadhl, Mo'tasam sent poison which Ummul Fadhl put in Imam's drink and gave it to him. Imam was martyred on 29th Dhulqa'ada at the age of 25 years and is buried near his grandfather in Kadhmain. His son the 10th Imam, Imam Ali un Naqi (a.s.) gave him ghusl and kafan.