During the first decade of November he served in the 3rd Regiment of the 2nd Brigade and took part in the battles of Lisów and Kostiuchnówka.

In 1928, after the first year of studies, he was sent to a two-year normal course at the École Supérieure de Guerre in Paris.

After completing his studies and obtaining the academic title of a certified officer on November 1, 1930, he was assigned to the Higher Military Academy as a lecturer.

There, he was introduced to the mass graves of Polish officers previously imprisoned in the camp in Kozelsk.

To protect himself from accusations of collaboration with the Germans, he prepared and buried copies of the memoranda on the camp grounds.

At the beginning of June 1945, he was promoted to colonel and transferred to Warsaw, where he became deputy chief of the 1st Department of the General Staff, and then chairman of the Polish Army Verification Commission.

In September 1945, he became head of the cabinet of the Minister of National Defense and was promoted to the rank of brigadier general.

In February 1947, he conducted inspections in the Voivodship Security Committees in Katowice, Kraków and Lublin, after which he proposed that the action of resettling the Ukrainians to the Western Lands should begin in the spring of 1947.

Mossor was the commander of the Operational Group "Vistula", established for the purpose of resettling the Ukrainian population.

When on 24 April 1947 the government presidium passed a resolution on the Operation Vistula, he received a nomination from the Minister of National Defence, Marshal Michał Rola-Żymierski, for the position of Government Plenipotentiary for the Operation of Resettlement of the Ukrainian Population and Combating the Ukrainian Insurgent Army.

As a result of this operation, the UIA lost about 70% of its personnel and its supply base in the Rzeszów region.

In recognition of his services, on 22 July 1947 he was promoted to the rank of Major General and took over the position of commander of the 5th Military District in Kraków.

At the end of 1949 he was transferred to the position of head of the Studies Office at the Ministry of National Defense.

He was accused of acting to the detriment of the Polish state and nation during World War II by preparing memoranda in which he "outlined the project of a fascist Poland under the protectorate of Hitlerite Germany", and after 1945 he conducted espionage activities directed against the people's state.

On August 13, 1951, in the so-called Trial of the Generals, the Supreme Military Court found him guilty of the charges and sentenced him to life imprisonment and degradation.

In prison he was subjected to brutal torture, but the investigators failed to break him and force him to incriminate others, primarily General Wacław Komar and Marshal Żymierski.

On December 10, 1955, the Council of State exercised the power of pardon and changed the life sentence to 15 years in prison.

The Supreme Military Court granted Mossor a one-year break from serving his sentence and on 13 December 1955 he left prison.

After the changes of the Polish October, in November 1956 he returned to military service, once again taking up the position of head of the Studies Office at the Ministry of National Defence.

He died of heart disease in Warsaw on 22 September 1957 and was buried at the Powązki Military Cemetery.