In the early years of the Ottoman Empire, all princes were required to work as provincial (sanjak) governors as a part of their training.
But after the death of his father, fearing for his life at the hands of his elder brother, he escaped to the rival Turkish beylik Karamanid territory.
In Karaman, he was encouraged by an Ottoman pasha named Şaraptar İlyas to rebel against his elder brother, new sultan Murad II.
Although unsuccessful in the siege of Bursa, the co-capital of the empire, Küçük Mustafa captured İznik (Nicaea of antiquity), another important city in the Anatolia.
İlyas betrayed Küçük Mustafa and changed sides, and İznik was easily captured by Murat's forces.