[1][2] Keizer joined Ajax Amsterdam at the age of 16, and two years later made his debut for the side, against Stormvogels on 1 April 1929.

Keizer was at once pitched into Arsenal's first team, and made his club debut against Blackpool on 30 August 1930.

With stiff competition coming from Arsenal's other keepers, Bill Harper and Charlie Preedy, Keizer lost his regular spot within the side.

His debut for Ons Oranje came in a qualifier against Belgium, a 4–2 win that took the Netherlands to the 1934 FIFA World Cup.

Keizer flew to London to ask his old club Arsenal for help, who obligingly donated a set of kits and footballs.

Keizer continued to repeatedly journey across the Channel, but in 1947, he was discovered to be smuggling British banknotes within leather footballs; he was fined 30,000 guilders and sentenced to six months imprisonment.