Promoted Wing Commander, he attended Staff College at UK, and immediately after independence became the AOC, Ambala in the rank of Group Captain. In 1949, promoted to Air Commodore, Arjan Singh took over the Air Officer Commanding of Operational Command, which later came to be known as Western Air Command. Arjan Singh had the distinction of having the longest tenure as the AOC of Operational Command, from 1949-1952 and again from 1957-1961. Promoted to Air Vice Marshal, he was the AOC-in-C of Operational Command. Towards the end of the 1962 war, he was appointed the DCAS and became the VCAS by 1963. He was the overall commander of the joint air training exercises "Shiksha" held between the IAF, RAF and RAAF.
On 1 August 1964, Arjan Singh took over as the Chief of Air Staff in the rank of Air Marshal, which became the pinnacle of this career. Arjan Singh was the first Air Chief who kept flying category till his CAS rank. Having flown over 60 different types of aircraft from Pre-WW-2 era Biplanes to the more contemporary, Gnats and Vampires, he also had flown in transports like the Super Constellation.
Arjan Singh's, testing time came in September 1965, when the subcontinent was plunged into war. When Pakistan launched its Operation Grand Slam, in which an armoured thrust targeted the vital town of Akhnur, he was summoned into the Defence Minister's office with a request for air support.
With a characteristic non-chalance, he replied "...in an hour." And true enough, the air force struck the Pakistani offensive in an hour. He led the air Force through the war showing successful leadership and effort. Though at a certain level, mistakes were made and planning could have been better, in all fairness, it must be said that the credit for thwarting Ayub Khan's grandiose plans to capture Kashmir is shared by the Army and the Indian Air Force, and Arjan Singh for leading the air force through the war.
Arjan Singh was awarded the Padma Vibhushan for his leadership of the air force, and subsequently in recognition of the air force's contribution in the war, the rank of the CAS was upgraded to that of Air Chief Marshal and Arjan Singh became the first Air Chief Marshal of the Indian Air Force. He retired in August 1969, thereupon accepting ambassador ship to Switzerland. He remained a flyer to the end of his tenure in the IAF, visiting forward squadrons & units and flying with them. Arjan Singh was a source of inspiration to a generation of Indians and Officers.
In recognition of his services, the Government of India conferred the rank of the Marshal of the Air Force onto Arjan Singh in January 2002 making him the first and the only 'Five Star' rank officer with the Indian Air Force.