Born in Bursa in 1390 to sultan Bayezid I and one of his wives, Devlet Hatun, Mehmed was the third oldest son of Bayezid. Along with his brothers from various mothers, including Süleyman Çelebi, İsa Çelebi, Mustafa Çelebi, and Musa Çelebi, Mehmed had the title Çelebi, meaning "gentleman".
On July 20, 1402, Bayezid was defeated in the Battle of Ankara by the Turko-Mongol conqueror and ruler Tamerlane. The brothers (with the exception of Mustafa, who was captured and taken along with Beyazid to Samarkand) were rescued from the battlefield, Mehmed being saved by Bayezid Pasha, who took him to his hometown of Amasya. Mehmed later made Bayezid Pasha his grand vizier (1413–1421).
After the battle, Mehmed fought with his brothers Süleyman, İsa, and Musa in the civil war known as the Ottoman Interregnum from 1402 to 1413, emerging victorious with the other three brothers dead. His other brother Mustafa would later emerge from hiding during Mehmed's reign and fight two failed rebellions against him and his son, Murat II.
After winning the Interregnum, Mehmed crowned himself sultan in the Thracian city of Edirne that lay in the European part of the empire (the area dividing the Anatolian and European sides of the empire, Constantinople and the surrounding region, was still held by the Byzantine Empire), becoming Mehmed I. He consolidated his power, made Edirne the most important of the dual capitals of Bursa to Edirne, and conquered parts of Albania, the Jandarid emirate, and the Armenian Kingdom of Cilicia from the Mamelukes. Taking many of his achievements into consideration, Mehmed is widely known as the "second founder" of the Ottoman Empire.