Hasan was born in the month of Sha'aban (January-February 625) or 15 Ramadan (1 March) in the 3^rd year after the emigration to Medina. His father (Ali) wanted to give him the name Harb (war), but his grandfather, Prophet Muhammad (pbuh) named him Hasan, which was not a common name during the Age of Ignorance, and he added the title Abu Muhammad. Prophet Muhammad recited the call to prayer in Hasan's ear, and he sacrificed an animal on the seventh day after the child's birth. Prophet Muhammad also told Fatima to give silver equal in weight to the weight of her hair to the poor. In some sources, it is reported that Abu Bakr said: "O Hasan, who is like the Prophet, not Ali," and Ali smiled at this expression.

Hasan, like his brother Hussein, did not get involved in the important events that occurred during the first caliphate. Hasan joined the Khorassan Campaign that was led by Said ibn As during the caliphate of Uthman. He was assigned by his father to protect Uthman against the rebels and to carry water to his home with his brother.

After his father was chosen as caliph, Hasan went to Qufa with Ammar ibn Yasir to persuade the people of Qufa to join his father against the rebels, Talha ibn Ubaydullah and Zubayr ibn Awwam. He was with his father in the Jamal Incident and the Battle of Siffin. After Ali was martyred, Ubaydillah ibn Abbas ibn Abdulmuttalib invited the people of Qufa to make a pledge of allegiance to Hasan. According to one account, they pledged allegiance to Hasan either on the same day or two days later. When asked about making a pledge to Hasan a short time before he had died, Ali had said: "I neither order it nor prohibit it." However, the Shiites believe that Ali had made Hasan his heir.

Muawiyah ibn Abu Sufyan was informed that Ali had been martyred and Hasan had now been chosen as caliph. He started intense activity to gather supporters and to get the people of Qufa on his side. He gathered an army composed of units from Syria, Palestine and Algeria led by Abdullah ibn Amir. Abdullah ibn Amir went to Madain and stood in front of the army of Hasan, which had left the city. He told them that Muawiyah had besieged Anbar, that they had no intention to fight, and that any soldiers who might take refuge with them including Hasan would be forgiven. In response to these words, it became clear that the majority wanted to avoid a war.  Hasan returned to Madain to inform Abdullah ibn Amir of the terms for handing over the caliphate to Muawiyah.

Hasan's terms were as follows:

1. No Iraqis were to be imprisoned for the sake of revenge.
2. All people would be able to live in safety, regardless of their nationality.
3. All crimes that had been committed were to be pardoned.
4. The ahwaz tax was to be paid to him annually.
5. His brother Hussein would be given 2 million dirhams.
6. The same kindness that was shown to the people of Abdushams (Umayya) was to be shown to the Hashemites.

Abdullah ibn Amir took Hasan's terms to Muawiyah. Muawiyah wrote the terms down, sealed them and returned them to Hasan (Rabi' al-awwal 25, 41 / July 29, 661).

Hasan was pleased that Muawiyah had accepted his terms. He informed Qays ibn Sa'd of the treaty and ordered him to hand over his authority to Muawiyah and to return to Madain. At the time, some people like Hussein and Hujr ibn Adi opposed the agreement between Hasan and Muawiyah, as they thought that Hasan had brought his fellow Muslims into contempt by making this treaty. However, Hasan did not change his mind; he went to Qufa from Madain with his men and confirmed the treaty with Muawiyah.

Because of this event, the 41^st year of Islamic history was called Amu al-Jamaa (the year of reunification). Although Hussein strongly opposed him, Hasan made a treaty in accordance with Prophet Muhammad's opinion (Bukhari, "Sulh", 9; "Fitan", 20), preventing blood shed between Muslims, and providing a peaceful and quiet environment for people, at least for a short time. Hasan went to Medina later with his family and lived a life removed from politics. According to some sources, he was poisoned by his wife Ja'da bint al-Ash'ath ibn Qays, who was encouraged to do this with the promise of marriage to Yazid ibn Muawiyah. Hasan died Safar 28, 49 (April 7, 669).

Before his death Hasan said that he wanted to be buried next to the tomb of Prophet Muhammad or in the Jannat al-Baqi cemetery, next to the tomb of his mother, if the first option was not possible. Marwan ibn Hakam refused the first option and Hasan was buried next to his mother in the Jannat al-Baqi cemetery after Said ibn As, the governor of Medina, led his funeral prayer.