On 15 July 1920 Mniszek was approved (1 April 1920) at the rank of major, "in the Cavalry, a group of former Polish Legions".
On 1 June 1921 he served in the Office of the Inner War Council, remaining on the register of Rokitniański Light Horseman Cavalry Regiment.
In 1923 he served in Division IV of the General Staff in Warsaw, as a director of the office, remaining on the register of the second Rokitiański Light Horseman Cavalry in Bielsko.
On September 24, 1939, Mniszek was commanded by General Tadeusz Malinowski to regroup KG NW region of Tulcza and Babadag.
On September 25, 1939, in Ploesta, Mniszek handed his task of abolishing the Headquarters of the Supreme Commander to his deputy Major Zygmunt Cierpicki, while he himself went to Bucharest.
From September 28 to December 23, 1939, while at the Polish Embassy in Bucharest, Mniszek was commanded by military attaché, lieutenant colonel.
His retirement followed allegations that were made against him by the Office of Registration of the Ministry of Military Affairs, headed by a cavalryman, Frederick Mally.