When Nazi Germany invaded Poland in 1939, the Polish design team escaped to the U.K. and resumed work together with Czech and British designers. The need for the Polsten was apparently mooted in June 1941. It went into service in March 1944 alongside the Oerlikon. Both the Oerlikon and the Polsten used similar 60 round drum magazines, although the Polsten could also use a simpler box magazine with 30 rounds. It remained in service into the 1950s.

Compared to the Oerlikon's 250 parts, the Polsten had only 119, but it matched the Oerlikon's effectiveness and reliability. The simpler design of the Polsten cannon made its production much cheaper. The cost of one Oerlikon cannon was about £350, while the cost of the Polsten was between £60 and £70. In January 1944, the 21st Army Group decided that only 20 mm Polsten guns would be used as a standard light anti-aircraft gun in place of the Oerlikon to simplify supply. It was used by the anti-aircraft platoons of some British infantry battalions during the Western European campaign of 1944-45. It equipped airborne units in the anti-aircraft role, and was employed in Operation Market Garden. The gun was placed on a wheeled mounting that could be towed behind a jeep.

The Polsten gun was used on armoured vehicles equipped with anti-aircraft guns that were based on the Cromwell/Centaur tank and for the Skink anti-aircraft tank.

The Polsten was also mounted on British LVTs and on early models of the Centurion tank, not coaxially with the main gun but in an independent mount on the left hand side of the turret. Various double, triple and quadruple mounts were developed. John Inglis Limited of Toronto, Ontario, in Canada produced many thousands of guns and some 500 quadruple mountings that saw limited service at the end of the war. These multiple mounts were both trailered and truck-mounted. Polsten Guns, magazines and ammunition boxes were also made in Australia by Holden's Woodville and Beverley plants during World War II. They were used by the Australian Army onshore and on small boats. Several prototype gun mountings were also developed but did not see service.

The origin of the name is not entirely clear. Some sources suggest Poland and the "Sten Company" to give Pol-sten, though the Sten gun was not made by a Sten Company. Official (United Kingdom) sources indicate the name to have been a compound based on Poland and the Royal Small Arms Factory Enfield in the same manner as the Bren gun (Brno + Enfield) or Sten (Shephard, Turpin + Enfield); also to reflect the gun design being mostly Polish (and the magazine mostly Czech) and the 8 Polish engineers in the design department. The "Sten" ending may also have linked in with the idea of the gun as a cheaper and quicker to produce weapon just like that gun was.

Former operators

•   Australia
•   Canada
•   Poland
•   United States
•   United Kingdom

• List of autocannon
• List of API blowback firearms
• Becker Type M2 20 mm cannon
• Polski Sten, the actual Polish derivate of the Sten gun
• Type 99 cannon

• Airborne Polsten gun
• Anti-Aircraft: Polsten 20mm Quad