Beamont's operational career began in 1939, flying Hawker Hurricanes with No. 87 Squadron RAF stationed at Lille in France with the air contingent of the British Expeditionary Force, scoring 3 'kills' against German aircraft. With the withdrawal of British forces from the continent following the fall of France he took part in the Battle of Britain, claiming 3 more 'kills', after which he was involved with nightfighting trials with the Hurricane. He was awarded a Distinguished Flying Cross in June 1941, and was posted to No. 79 Squadron RAF, although he was court-martialled for transporting a WAAF to a dance at another RAF station in his single-seat Hurricane, and was severely admonished.

In December 1941 he was attached to Hawker's at Langley as a Production and Experimental Test Pilot. In July 1942 he was back on operational flying, joining No. 609 squadron flying Hawker Typhoons and subsequently promoted to Squadron Commander. As Commanding Officer of one of the first Squadrons to operate the new and technically troublesome Typhoon, Beamont was instrumental in arguing for keeping the aircraft in RAF service against increasing establishment resistance while he assisted Hawker's in resolving the type's airframe and engine problems. His confidence in the Typhoon was vindicated as the aircraft eventually became the RAF's most successful ground-attack aircraft during 1944-5.

In 1943 he returned to Hawker's as a test pilot, performing experimental testing of the Hawker Tempest alongside the Hawker's test pilot, Bill Humble. In 1944 prior to D-Day, he again resumed operational flying, this time forming the first Tempest Wing (No. 150) with the rank of Acting-Wing Commander, the Wing accounting for three Me 109 over the invasion beaches shortly after D-Day without loss, two credited to Beamont. At this time the Wing were switched to intercepting V-1 over Kent, shooting down 638, Beamont accounting for 32 of the unpiloted-flying bombs himself. On October 2, 1944, now based on the continent at Volkel, Holland, he achieved his ninth and final kill of the war when he shot down a Fw 190 near Nijmegen. On October 12, while attacking a heavily defended troop-train near Bocholt on his 492nd operational mission he was shot down, becoming a Prisoner of War. Confined firstly to Stalag-III at Sagan, near Breslau, then to Stalag III-A at Luckenwalde, near Potsdam, he remained a PoW until the end of the war in Europe, being finally repatriated in late May 1945 after being further detained with the other former POWs by the Russians.

With the end of hostilities his planned transfer to the Far East in command of a Wing of Tempest IIs was cancelled and he applied for a permanent commission. The eventual offer of a permanent commission coincided with his being offered a position as a test pilot; he resumed his career as a test pilot, performing the initial test flying of many notable aircraft, including the Canberra and Lightning (see radio interview details below), as well as the later-cancelled BAC TSR-2. He was also the first British pilot to fly supersonic (i.e. faster than the speed of sound). He subsequently went on to become a director of the Warton division of BAC, later BAe, as a Director of Flight Operations. From 1970 he was responsible for the international testing of the Tornado MRCA. In 1979 he retired, devoting himself to writing and contributing to various aeronautical publications. He died on 19 November 2001 at the age of 81.

Beamont has the distinction of being the first pilot to make a double-atlantic crossing by jet, when on 26 August 1952, flying Canberra B.5 VX185, he flew from Aldergrove to Gander and then back again to Aldergove, in 10 hours 3 minutes.

He was posthumously awarded the Belgian Croix de guerre.