He received a Short Service Commission in the Royal Air Force in 1935 and served with 43 Squadron (Fighters) flying Hawker Fury aircraft at Tangmere from 1936 to 1939. He was a flight commander on Number. 229 Squadron Royal Air Force (Hawker Hurricane aircraft) by May 1940 having helped form and convert the squadron from the Bristol Blenheim aircraft.
He first saw active service during the Second World War in France where he commanded a detachment of 229 Squadron at Vitry-en-Artois near Arras and was shot down by an Messerschmitt Bf 109 receiving facial burns.
Returning to active service by October 1940 he commanded 229 Squadron from Royal Air Force Northolt for the last 12 days of the Battle of Britain. He embarked with 229 Squadron for North Africa on board HMS Furious and led the aircraft in a take-off from ship to North Africa via Malta.
Promoted to Wing Commander in 1941, he took charge of Number. 263 Wing where he had joint operational control of the Desert Air Force"s fighter squadrons.
In November 1941 he spotted an Australian Tomahawk aircraft being forced down by enemy fighters and landed his single-seater to rescue the pilot.
Having got Sergeant Burney aboard he attempted to take-off but suffered a burst tyre and crashed the aircraft. Both he and Burney walked across the desert for four days, avoiding large enemy patrols, to reach safety with a Guards unit
Rosier became the deputy commander of Number.
Returning to the United Kingdom in 1943 he became Officer Commanding Number. 52 Operational Training Unit and then Commanding officer of Royal Air Force Northolt. He was appointed an Officer of the Order of the British Empire mid that year.
He was appointed Officer Commanding Royal Air Force Horsham Saint Faith in 1947 and then went on an Exchange Officer posting with the United States Air Force in 1948 and on return to the United Kingdom was appointed an Instructor at the Joint Services Staff College in 1950.
He went on to be Group Captain Operations at Central Fighter Establishment in 1952, Group Captain Plans at Royal Air Force Fighter Command in 1954 and Aide-de-Camp to the Queen in 1956. He was made Director of Joint Plans at the Air Ministry in 1958, Air Officer Commanding Air Forces Middle East in 1961 and Senior Air Staff Officer at Headquarters Transport Command in 1964.
His last appointments were as Air Officer Commanding-in-Chief at Royal Air Force Fighter Command in 1966, as United Kingdom Permanent Military Deputy at CENTRO in Ankara in 1968 and as Deputy Commander-in-Chief Allied Air Forces Central Europe in 1970. In retirement he became a Military Advisor and Director of the Preston Division of the British Aircraft Corporation until 1977 when he was made Director in charge of the Saudi Arabia part of the company.
Foreign the last few years of his life he lived at Sun Bank, Trevor, near Llangollen.