He was released from Soviet captivity on amnesty following the Sikorski–Mayski agreement as an officer of the Polish Army in the East by efforts of (commander-in-chief general Władysław Anders) in 1942.

Zaleski, despite earlier promises, declined to step down after seven years in office (1954), which caused formation of the Council of Three, an opposition body, recognized by a part of the emigres as a collective head of state.

He also strongly refused to acknowledge annexation of the eastern areas of the Second Polish Republic by the USSR after World War II.

Ostrowski died in London and was buried in a Polish military cemetery in Newark-on-Trent, along with August Zaleski and Władysław Raczkiewicz, other past Presidents in exile.

In November 2022, the trio's remains were moved from the cemetery at Newark-on-Trent in England to the mausoleum for emigree presidents at the Temple of Divine Providence in Warsaw.