No. 133 Squadron was formed on 1 August 1941 at Coltishall and assigned the squadron code MD. George A. Brown, an Englishman, transferred from No. 71 to become squadron leader. On 16 August the squadron moved to Duxford where it was brought up to operational strength. September 1941 was spent at Colly Weston and Fowlmere training in Hurricane IIBs and flying convoy patrol in the North Sea. In early October the squadron was transferred to Eglinton, Ireland to complete training & fly convoy patrol duty over the North Atlantic. Tragically 4 pilots (Stout, McCall, White, and Mamedoff) were killed enroute to Eglinton when they ran into a mountain in bad weather and two more were killed (Bruce, Coxetter) in accidents within two weeks of their arriving in Ireland. Late in October Eric Thomas, another Englishman, replaced Brown as Squadron Leader. On 28 October the first Spit II's arrived to replace the Hurricanes that were lost in crashes.
2 January 1942 brought a transfer to Kirton-in-Lindsey where they received war weary Spit VAs. 133 saw it's first action against the Germans on 5 February when on convoy patrol they engaged in a series of combats with several Do 217s and shot down one. Shorty thereafter they received new Spitfire VBs. The months of February, March & April were spent in rather monotonous convoy partol duty with the occasional sweep into France.
It wasn't until April 26th that No. 133 again met the Germans while on a fighter sweep, and Carroll McColpin, who had transferred from No. 71 in January, shot down a FW 190 near Boulogne. On the 27th, during an escort mission to Ostend, William Baker and Robert Pewitt were each credited with probably destroying a FW 190, however Walter Wicker was killed. By late April 133 began flying squadron strength night interceptor missions. On the 29th Eric Doorly damaged a Dornier over York during one of these night missions. On 7 May 1942 133 was transferred to Biggin Hill, which was in Group 11 sector, and the pilots finally got the action they were looking for. Almost daily missions were flown including bomber escort and fighter sweeps and many of the pilots had combat with Me 109's. On 17 May No. 133 made a fighter sweep over the Abbeville area with F/Lt McColpin claiming a Me 109F destroyed and another probable and P/O Morris claiming a probable. Two days later on the 19th Carter Harp shot down two Fw 190s and Moran Morris and George Sperry each shot down a Me 109 in fierce fighting over Le Treport. The rest of May brought intense combat over northern France with many claims of damaged Me 109's, and a FW 190 downed by Edwin Taylor near Dieppe on 31 May, at the cost of 4 more pilots killed (Florance, Pewitt, Morris, Ford).
In June Don Blakeslee transferred from No 401 and managed to damage a Ju 88 on the 27th over the French coast. On 31 July, during a bomber escort mission to Abbeville (Circus No. 201), No. 133 engaged in furious combat resulting in 3 destroyed and one probable while losing Harp, King, and Eichar. P/O Edwin "Jessie" Taylor accounted for 2 of the kills (109F and 190) & and P/O William Baker was credited with a FW 190 destroyed near Le Crotoy. Taylor recalled:
"When we were jumped by the Germans, I immediately latched on to an FW 190 and shot him down. At the same time there evidently was a German on my tail. My wing man, Carter Harp, called to warn me but I never did hear him and he was shot down at the same time. I was hit badly with a bullet grazing my head, which blinded me, one through my foot and others all over the airplane. I pulled up figuring that I would stall and when I felt that I would get out. Amazingly my eyesight came back and I went down on the deck after three other Germans. I shot an Me 109 which was having a terrible time staying in the air and I am sure it crashed before getting back to France. Down on the deck I engaged two more FW 190s and, although I did not see them crash, Squadron Leader Thomas and William Baker saw the two crash."At the end of July No. 133 moved to Gravesend and again on 14 August to Lympne. Don Blakeslee took over as Squadron leader and downed an FW 190 on 18 August near Sangette when No 133 flew a Rodeo mission with the American 307th flying Spits and No 65 Sqdn. On 19 August the squadron flew 4 missions as part of "Operation Jubilee", the covering of the invasion of France at Dieppe, and tallied 6 destroyed, 2 probables, and 8 damaged.
Blakeslee got "busted", having been caught with two WAAFs in his room, so Carroll McColpin took over command of the squadron when he returned from leave in the States. Early in September the squadron moved back to Biggin Hill and received new Spitfire Mk IXs. On the 6th the squadron escorted B-17s to Meaulte and mixed it up with Fw 190s losing Doorly and Gudmundson. On the 7th 133 escorted B-17s to Rotterdams's shipyard where William Baker was credited with a 190 probable. The last score for the squadron came on the 16th when C. Miley was credited with damaging a FW 190 in a scrap near Deal. On 23 September the squadron moved to Great Stampford. Three days later on 26 September catastrophe struck. Known as the Morlaix mission, 133 was tasked with bomber escort and were to rendezvous with B-17s over the Bay of Biscay. Strong winds blew the unit further south then they realized and short of fuel they let down directly over Brest, France. Six of the squadron were shot down & taken prisoner, four were killed, one bailed and evaded capture, and one crash landed in England. The entire squadron was wiped out. During it's existence 133 Squadron was credited with 14 1/2 aircraft destroyed. On 29 September 1942 what was left of the unit transferred to the USAAF and became the 336th Squadron of the Fourth Fighter Group.