He was commissioned into the Royal Air Force as Pilot Officer on 17 September 1945. During his service he was posted at numerous bases including the Royal Air Force Base Kohat in 1946. He also flew a variety of aircraft including Spitfires.
In 1946 Callaghan belly landed his aircraft in Kohat airfield.
The aircraft engine had caught fire after an oil leak during landing practice. The aircraft was later written off in 1947.
In 1947 at the partition of India, he opted to serve with the Pakistan Air Force (PAF). On 1 September 1951 a PAF two seater Fury aircraft caught fire.
Now Flight Lieutenant Callaghan was the second pilot on that flight.
By 1969 he had risen through the ranks of Squadron Leader and Wing Commander. When a C-130 aircraft of the PAF while coming back to Pakistan from China had gone missing, Group Captain Callaghan was part of the PAF team investigating the crash. Air Commodore Callaghan was in 1971, the PAF Chief Inspector, in charge of the verification of Pakistani claims of enemy airplane kills.
Air Vice-Marshal Patrick Callaghan of the Pakistan Air Force’s Eastern Air Force Command was present on 16 December 1971 when the Instrument of Surrender was signed at Dhaka’s Ramna Race Course by Lieutenant General Jagjit Singh Aurora of the Indian army and Lieutenant General Amir Abdullah Khan Niazi of the Pakistan Army to mark a formal end to the Indo-Pakistani war of 1971.
Callaghan was one of the signatories to the instrument of surrender. He was also the Deputy Chief of Air Staff of the Pakistan Air Force in 1971.