Born on 9 June 1885 in Gąbin, Congress Poland, Sławoj Składkowski was raised in a family with strong patriotic beliefs.

Felicjan was one of six children: he had one brother, Bożywoj, and four sisters, Dobrosława, Tomiła, Mirosława and youngest Wincencja, who died in infancy.

To continue the studies, he left for Austrian Galicia, and in March 1906 joined the Medical Department of the Jagiellonian University in Kraków.

After the outbreak of World War I, together with one of the organizers of the Rifle Association in the Dąbrowa Basin, Stanisław Zwierzyński, he joined the Polish Legions stationed in Miechów on 13 August 1914.

On 16 July 1917, during the Oath crisis, Składkowski, who officially was considered a Russian subject, was released from the service in the Legions.

In November 1918, Składkowski, wearing his Legions uniform, disarmed German soldiers in the region of Zagłębie Dąbrowskie.

On 3 February 1919, Składkowski was named chief of staff of Colonel Rudolf Tarnawski, commandant of Będzin Military District.

On 7 February 1920, he was appointed to the post of manager of the organizational section of the Ministry of Military Affairs' Medical Department.

During the May Coup, Składkowski supported Józef Piłsudski, and, on 13 May 1926, General Gustaw Orlicz-Dreszer appointed him government commissar for Warsaw.

On 2 October 1926, Składkowski, who by then was widely known as an avid supporter of Józef Piłsudski, was nominated the Minister of Internal Affairs.

On 27 March 1928, during a session of the Sejm (Polish parliament), communist envoys and senators interrupted the speech of Marshal Piłsudski, shouting out anti-government slogans.

On 1 January 1930, he returned to the armed forces and was appointed Deputy Minister of Military Affairs and head of the Army Administration.

As a minister, who was a physician by profession, Składkowski was very concerned about the poor state of hygiene on Polish farms and estates in the countryside.

Personally, Felicjan Sławoj was extremely close with Józef Piłsudski, often being invited with his wife to the marshal's parties or dinners, and was one of the first people to receive the sad news about his unexpected death in 1935.

After Piłsudski's death, the camp of his followers divided into factions, including the supporters of President Ignacy Mościcki, and those who favoured Marshal Edward Rydz-Śmigły.

Among the members of his government were politicians of different factions, such as Eugeniusz Kwiatkowski and Minister of Foreign Affairs Józef Beck.

Składkowski himself tried to stay away from any political conflicts, and concentrated his efforts on improving the state administration, especially police force and civil services.

He frequently toured Poland, visiting schools, police stations, manufacturing plants and farm estates.

Polish officials pursued the idea of Jewish emigration at the League of Nations and in bilateral talks with France and other relevant powers.

On 1 September 1939 at 4:30 a.m., Składkowski, who spent the night in the complex of the Ministry of Internal Affairs, received a phone call from Kraków, informing him about the German invasion of Poland and a brutal attack in the border town of Chojnice.

The next day he gave a speech in the Sejm, expressing his hope that Poland would defeat the Third Reich and win the war.

On 17 September, upon hearing of the Soviet invasion of Poland, Składkowski crossed the Romanian border on the Czeremosz River bridge near Kuty.

There, together with Eugeniusz Kwiatkowski, he wrote a letter to President Raczkiewicz (9 October), demanding a permission to leave Romania.

In late 1940, Składkowski was sent to the Reserve Center of the Polish Independent Carpathian Rifle Brigade (General Stanislaw Kopański).

On 25 January, upon request of General Kordian Józef Zamorski, he was named sanitary inspector of the local units of the Polish Army.