On 27 April 1917 he volunteered for the Polish I Corps in Russia commanded by the general Józef Dowbor-Muśnicki: injured in June 1918, he was sent home to his family estate.

After working two years as an accountant in a sugar refinery in Stepanówce, near Vinnytsia, he joined anew the Polish army in 1920, and participated to the Battle of Warsaw for which he was decorated.

Still serving in different army units, lived in 1921 in Poznań, in 1922–1923, in Toruń and in May 1923, he moved to Bydgoszcz to be employed as a building draftsman for the District Engineering Management (Polish: Kierownictwo Rejonu Inżynierii i Saperów).

Jan Kossowski asked to be transferred from Toruń to Bydgoszcz so as to start learning at the State School of Art Industry.

Demobilized, he initially found a job in a water and sewage installation company owned by engineer Józef Piecek, and subsequently worked in the office of architect Karl Schaum from April 1940 to May 1943:[1] during this period, he supervised the construction of arcades at the ground floor of opposite tenements at 2 Jagiellońska street in Bydgoszcz and 2 Focha street.

After the liberation, he was appointed by Polish municipal authorities in January 1945, as the head of the Urban Planning and Development Department and as the city architect.

In June 1953, he submitted a design for the philharmonic building; though not endorsed by the selection committee, his concept was pretty close to Stefan Klajbor's one realized in 1954–1958.

Subsequent projects, built after the Great Depression, are characterized by richer forms, with more expensive linings and a closer attention to details.