Mustafa I: Ottoman Sultan from 1617 to 1618 and 1622 to 1623

Mustafa I, also known as Mustafa the Saint (Veli Mustafa) during his second reign and Mustafa the Mad (Deli Mustafa) by later historians, was the Sultan of the Ottoman Empire from 1617 to 1618 and from 1622 to 1623.

Mustafa was the younger brother of Ahmed I (1603–1617) and was born in the Manisa Palace. Halime Sultan, an Abkhazian lady, was his mother. Before 1603 it was common for an Ottoman Sultan to have his brothers executed soon after assuming power. When Ahmed I, at thirteen years old, was crowned in 1603, he spared the life of Mustafa, then twelve years old. Mustafa's survival may have been aided by the influence of Kösem Sultan (Ahmed's favourite concubine). He may have intended to prevent Osman, Ahmed's first-born son, from inheriting from another concubine. If Osman were to become Sultan, he would most certainly try to assassinate his half-brothers, Ahmed and Kösem's sons. (This possibility became a reality when Osman II killed his brother Mehmed in 1621.) Foreign embassy reports, on the other hand, claim that Ahmed liked his brother. Mustafa, his mother, and Grandmother Safiye Sultan remained at the Old Palace until Ahmed died in 1617.

Ahmed's death presented the Ottoman Empire with a problem it had never faced before. The Sultanate was now open to multiple princes, all of whom resided in Topkapı Palace. Mustafa was enthroned instead of Ahmed's son Osman by a court party led by the  Şeyhülislam Esad Efendi and Sofu Mehmed Pasha (who represented the Grand Vizier when he was absent from Constantinople). Sofu Mehmed believed that Osman was too young to be crowned without provoking widespread opposition. Chief Black Eunuch Mustafa Agha raised an objection based on Mustafa's mental health concerns, but he was overruled. Mustafa's ascent established a new seniority-based succession system that would remain until the Empire's demise. It was the first time in Ottoman history that an Ottoman Sultan was replaced by his brother rather than his son. His mother, Halime Sultan, rose to the position of Valide Sultan and regent, wielding immense influence. Because of Mustafa's mental state, she took on regent and exerted power more directly. Mustafa's mental health was thought to improve with regular social contact, but his conduct remained odd. He yanked his viziers' beards and yanked their turbans off. Others saw him flinging coins at birds and fish.

At the Topkapı Palace, Mustafa was never more than a tool of court cliques. After a brief reign, Mustafa was overthrown in 1618 by a palace party favouring his young nephew Osman II (1618–1622), and he was transported back to the Old Palace. He was given a second opportunity during the dispute between the Janissaries and Osman II. After Osman II was deposed and assassinated by a Janissary revolt in 1622, Mustafa was reinstated to the throne and reigned for another year.

Nonetheless, Baki Tezcan claims that there is little evidence to prove that Mustafa was mentally ill when he ascended to the throne. On the contrary, Mustafa "made several trips to the arsenal and the naval docks, examining various types of armaments and taking a keen interest in the army and navy's munitions supply." "Suggested that Mustafa was interested in conducting the Safavid campaign himself and was contemplating the notion of wintering in Konya for that purpose," according to one of the French ambassador's reports. Furthermore, one contemporary observer explains the coup that omits Mustafa's inability. According to Baron de Sancy, the deposition resulted from a political plot involving Grand Admiral Ali Pasha and Chief Black Eunuch Mustafa Agha. They were both enraged by the former's expulsion from office following Sultan Mustafa's ascension. Thus, they may have spread tales about the Sultan's mental illness after the coup to give it legitimacy.

He began his reign by assassinating everyone involved in Sultan Osman's assassination. Hoca Ömer  Efendi, the commander of the rebels, the kızlar Agha Suleiman Agha, the vizier Dilaver Pasha, the Kaim-makam Ahmed Pasha, the defterdar Baki Pasha, the segban-bashi Nasuh Agha, and the general of the janissaries Ali Agha were all chopped to pieces. During his reign, the appellation "Veli" (meaning "saint") was applied to him. Due to his worsening mental health, Mustafa was used as a puppet by his mother and brother-in-law, the grand vizier Kara Davud Pasha. He believed Osman II was still alive, and he was seen exploring the palace for him, banging on doors and pleading with his nephew to relieve him of the weight of rule. According to English Ambassador Sir Thomas Roe, "the current emperor is an idiot," He was compared negatively to his predecessor. His mother, Halime Sultan, was the Ottoman empire's de facto co-ruler as Valide Sultan.

The conflict between the Janissaries and the sipahis (Ottoman cavalry) caused political instability, followed by the Abaza insurrection, which happened when the governor-general of Erzurum, Abaza Mehmed Pasha, planned to march to Istanbul to revenge Osman II's assassination. The authorities attempted to stop the fight by assassinating Kara Davud Pasha, but Abaza Mehmed pressed on. Mustafa's mother was persuaded by clerics and the new Grand Vizier (Kemankeş Kara Ali Pasha) to allow her son's deposition. On the condition that Mustafa's life is saved, she consented. Murad IV, the 11-year-old son of Ahmed I and Kösem, was crowned on 10 September 1623. Mustafa's mother's request to have him saved from execution was granted in exchange for her permission to his deposition. Mustafa and his mother were taken to the Eski (ancient) Palace.

According to one story, Mustafa was executed on his nephew. Sultan Murad IV on 20 January 1640 to put an end to the Ottoman dynasty and prevent his mother, Kösem Sultan, from taking power. According to another story, he died of epilepsy due to being imprisoned for 34 of his 48 years of life. He is buried in the Haghia Sophia's courtyard.