Zachwatowicz was born in Gatchina. He studied how to build things at a university in Saint Petersburg and finished school for architects in Warsaw in 1930. He got a special award for his work in architecture in 1971.
He became a teacher at the Warsaw University in 1946, and he had been working there since 1925. He was also in a group of smart people called the Polish Academy of Sciences starting in 1952. In 1967, he joined a special group in Paris for architects. He also helped take care of old buildings in Poland from 1945 to 1957. He was the leader of a group that decided how to fix old buildings starting in 1971, and he was in charge of fixing the Royal Castle in Warsaw.
Zachwatowicz made the Polish restoration service better and helped it grow. When Poland was occupied during World War II, he taught and saved old buildings. Starting in January 1945, he helped run an office to fix Warsaw.
After World War II, Zachwatowicz and his team fixed or rebuilt many old buildings in Gdańsk, Poznań, and Wrocław using his ideas. One of his biggest achievements was rebuilding Saint John's Cathedral in Warsaw in 1960.
In 1952, he became a corresponding member, and in 1958 a full member of the Polish Academy of Sciences.
In 1964, he signed the Venice Charter defining the principles of conservation and restoration of architectural monuments.
Jan Zachwatowicz, together with Jan Bogusławski, was the initiator and author of the reconstruction of the Royal Castle in Warsaw in 1971–1984.
He was a smart person with more than 200 important writings. He died in Warsaw when he was 83 years old.He was buried at the Powązki Cemetery in Warsaw.
The initiator of the monument's construction was the Society of Friends of Warsaw, which first undertook efforts in this direction in 2017. On December 8, 2020, the Warsaw City Council adopted a resolution regarding the erection of a monument to Jan Zachwatowicz at the Castle Square. The execution of the works was overseen by the downtown Management of Public Areas. The monument is located at the Castle Square, at the entrance to the so-called Piotr Biegański inter-wall area. The monument was unveiled on March 4, 2021, on the 121st anniversary of Jan Zachwatowicz's birth. The unveiling of the monument was carried out by Rafał Trzaskowski, the Mayor of the capital city of Warsaw, Ewa Malinowska-Grupińska, the Chairwoman of the City Council, and Krystyna Zachwatowicz-Wajda, the professor's daughter.
The author of the monument is the Krakow sculptor Karol Badyna. The bronze statue depicts Professor Jan Zachwatowicz facing the Royal Castle, to the reconstruction of which the conservator devoted the most work and effort. Zachwatowicz has his left hand in his pocket and is shown walking through the Old Town. At the base of the monument, there is a plaque that reads "1900–1983 JAN ZACHWATOWICZ ARCHITECT OF REBUILT WARSAW".