Hajji Beg is first mentioned in 1358 or 1359, when he participated in the overthrow of the Qara'unas ‘Abdullah, who was effectively in control of the southern Chagatai Khanate.
'Abd Allah, who had recently taken power, was young and inexperienced, and his move to Samarkand threatened Hajji Beg, whose territories were centered in the nearby city of Kesh.
Together with another tribal leader named Buyan Suldus, Hajji Beg removed him from power and killed his puppet khan Shah Temur.
Buyan Suldus was then raised to 'Abd Allah's former position of amir.
Buyan Suldus' refusal to enforce his authority, as well as a continuing state of chaos within the Chagatai ulus, leads the Khan of Moghulistan, Tughlugh Timur, to invade in 1360.
Most of the Chagatai leaders do not oppose the invasion; many of them take the opportunity to pillage each other's lands.
The Yasa'ur Hajji Mahmudshah, whose tribe borders the Barlas, decides to raid their territory with a Moghul army.
Hajji Beg decides at first to resist, but seeing that the Moghuls are much stronger, he flees to Khorasan.
During his retreat, Hajji Beg is accompanied by Timur, a member of a prominent Barlas family, who are Mongols that had been Turkified.
At the age of eight or nine, Timur and his mother and brothers were carried as prisoners to Samarkand by an invading Mongol army.
In his childhood, Timur and a small band of followers raided travelers for goods, especially animals such as sheep, horses, and cattle.
When Hajji Beg’s party reaches the Oxus River, Timur asks to return to Kish so that he can maintain order within the Barlas region.
Timur's prominent standings within the Barlas and with several members of the Moghul elite, however, result in Tughlugh Temur's appointing him as the ruler of the Barlas region.
Timur now aligns himself with tribal leaders that are hostile to Hajji Beg, eventually throwing his support behind Amir Husayn, a nephew of 'Abd Allah and the current leader of the Qara'unas.
In the meantime, the Moghuls abandon the region and head back to Moghulistan.
Soon afterwards, Hajji Beg returns to the Chagatai ulus in an effort to regain control of the Barlas.
He goes to the ruler of the Jalayir, Bayazid, and together they attack the Yasa'uri.
When Timur hears of this, he moves his army in support of the Yasa'uri.
The two sides meet in battle, and while its outcome is disagreed upon, it causes the Barlas emirs as well as the army to defect back to Hajji Beg.
As a result, Timur is constrained to submit to Hajji Beg.
The Barlas and Jalayir then again attack the Yasa'uri and defeats them; this victory secures Hajji Beg's position as leader of his tribe.
In the spring of 1361 Tughlugh Timur again invades the ulus.
Knowing that both Bayazid Jalayir and Buyan Suldus had decided to pledge their allegiance to the khan, Hajji Beg plans to do so as well.
Tughlugh Timur's execution of Bayazid, however, prompts him to change his mind.
He goes to Kish to gather troops, then crosses the Oxus into Khurasan, but there he is killed by a group of Turks.
Tughlugh Timur now gives Timur command of Kish a second time, but he will lose it soon afterwards.
