In 1936 a few Fokker D.XXIs were used by the Spanish Republic. Although the order by the ML-KNIL was cancelled, the Luchtvaartafdeling (Dutch Army Air Force before World War II) placed an order of 36 aircraft, which were all delivered in time to participate in the war against the Germans in May 1940. The Fokker D.XXI, although much slower and more lightly armed than the Bf 109, performed surprisingly well in dogfights, due to its maneuverability. It was also one of the few aircraft that could follow a Stuka bomber into its dive. Nonetheless, the numerical inferiority of the Luchtvaartafdeling compared to the Luftwaffe resulted in the destruction of most Dutch Fokker D.XXI fighters during the campaign. Some were captured during and after 15 May, but their fates, apart from their capture, are unknown.
The Fokker D.XXI performed better and for much longer in the Finnish Air Force, which had acquired a number of licence-built fighters prior to the start of the Winter War. Against the aircraft of the Soviet Air Force, the Fokker was more evenly matched, and its rugged design with a radial engine and fixed undercarriage made it very suitable for Finnish conditions. Later in the war, as newer models of Soviet fighters appeared, the Fokker D.XXI was underpowered and too lightly armed (with only four 7.92 mm/.312 in machine guns) to compete. Plans to arm the Fokkers with 20 mm cannons were dropped and only one fighter was armed as such (two 20 mm cannons and two 7.92 mm/.312 in machine guns). Another fighter was equipped with retractable landing gear, but due to less than anticipated performance improvement, wasn't continued in the series. During the Continuation War (1941–44) the Finnish State Aircraft Factory (Valtion Lentokonetehdas, VL) also built some 50 D.XXIs with the Swedish-built Pratt & Whitney R-1535 Twin Wasp Junior as the Bristol Mercury was in short supply. These can be identified by their longer cockpit glazing, smooth cowl, and large ventral air intake under the cowl. The fixed undercarriage lent itself to both unimproved runways and conversion to skis for winter use, both of which were advantages in the Finnish theater.
Several Finnish Air Force pilots became fighter aces with the Fokker D.XXI. The top scoring Fokker ace was Jorma Sarvanto who obtained 12 5/6 victories with the type. Many other future aces scored at least one victory with the Fokker. The highest scoring airframe was FK-110, with 10 victories. This aircraft survived the war and is on display at the Central Finland Aviation Museum.