He worked primarily in logic and foundations of mathematics and is perhaps best remembered for the Mostowski collapse lemma.

He became an accountant after the German invasion of Poland but continued working in the Underground Warsaw University.

In 1954 Mostowski was awarded by Knight's Cross of the Order of Polonia Restituta and in 1963 elected a real member of the PAS.

After the World War II he supervised Rasiowa's both master and doctoral theses in logic and the foundations of mathematics.

[2] With Krzysztof Kurdyka and Adam Parusinski, Tadeusz Mostowski solved René Thom's gradient conjecture in 2000.