[1][2] His father Sofroniusz Skrebeciowicz de Sielecki (1862–1908) was civil servant at the Imperial Royal Austrian State Railways, stationed in various places throughout Galicia.

Both became orphans at an early age and were raised by their maternal aunt Jozéfina and her husband Adolf Skrzyszewski, who was also employed at the state railway.

[3] Klemens Sielecki went to primary school in Chernivtsi and the Imperial and Royal Gymnasium V in Kraków and received his matriculation in Stanisławów in 1921.

In 1935 he was highly involved in the development of the construction of the diesel hydraulic railway car called "Luxtorpeda", which was a technical innovation in its time.

Due to his knowledge of foreign languages he was repeatedly sent abroad for business purposes pertaining technical aspects and co-operation.

In this time, with the consent of the general director Guido Sanchez de la Cerda, he was involved in protecting his Jewish co-workers from Nazi persecution.

After Poland was liberated, this documentation proved vital for the re-construction efforts of the national railway industry, especially the diesel and electrical locomotives.

[1] Although he refused to join the Polish United Workers' Party (PZPR) throughout his life, he nevertheless was decorated for outstanding professional and social work in 1955 with the Medal of the 10th Anniversary of People's Poland and in 1967 with the Knight's Cross (Krzyż Kawalerski) of the Order of Polonia Restituta.

The cousin of his wife Wanda was the investigation judge against Nazi crimes Professor Jan Sehn, who was on very friendly terms with the Sielecki family.