Qajar entered military service on July 19, 1879, in the 164th Infantry Regiment Zagatala. In 1883, he was promoted to Second Lieutenant. On November 20, 1886, he was transferred to the 2nd Battalion plastun of the Kuban Cossacks. On May 9, 1902, Qazar was promoted to yesaul. On April 16, 1909, he became the Army yesaul-commander of the 9th Battalion of the Kuban plastun.
During World War I, Qajar fought on the Austrian front. He was awarded the Order of St. Anne 2nd Class with swords, the Order St. Vladimir 3rd Class with swords, and the Order St. Stanislaus 3rd Class. On April 25, 1915, he was promoted to colonel. Because of a severe leg wound, Amanullah was sent to the rear for treatment. In 1916, he returned to the front. Along with his battalion, he fought near the village of Marhonovka. Qajar captured the enemy trenches and destroyed the enemy's manpower. During the operations on November 5, 1916, he was awarded the Saint George Sword. He received the rank of Major General in 1917. After the February Revolution, he lived in Tbilisi and Shusha.
After the Azerbaijan Democratic Republic created its first army as a newly established state, Qajar filed a report on December 1, 1918, to the newly established Ministry of War with a request of admission to their armed forces. In March 1919 he was a part of the emergency diplomatic mission of the Republic of Azerbaijan and participated in the Persian government in Tehran. He served as chairman of the central military service presence. On January 27, 1920, Major General Qajar was appointed as the deputy chief of the 1st Infantry Division of the ADR and head of the garrisons in Khankendi and Shusha. He participated in fighting the attacks of the Armenian armed forces on the military units of the ADR on 22-23 March 1920.
After the fall of the ADR in connection with the repression of the Bolsheviks, Amanullah was forced to leave for Iran. Living in Tehran, he taught at the military school and participated in the formation of the Iranian army. He was a deputy of the Majlis of Iran, and the chairman of the society of the Iranian-Soviet friendship. He died in 1937 in Tehran.
