Siepmann grew-up in an Evangelical family and through his father's business relations, was able to secure a commercial apprenticeship at J.C. Söding & Halbach, a steel manufacturing company, in Hagen.

He initially worked there until being deployed by his brother Emil Siepmann to lead the commercial department of Peters & Company in Warstein.

His first public endowment was in 1907, when he contributed 5 Goldmark (approximately $3,200 in 2024), to the construction the Equestrian Monument in Windhoek (then German South West Africa).

In 1921, the Siepmann brothers donated 250,000 Papiermark (approximately $800,000 in 2024) for the construction of the Warburg children's home, an orphanage, on the North German island of Norderney.

Additionally, Siepmann was among the founders and patrons of the Kriegsstiftung des Kreises Arnsberg, which provided aid to families affected from World War I.

His father in-law hailed from a well-established family of lawyers, judges and politicians in Stolberg (Harz) and was primarily known for his industrial activities in the Ruhr valley during the late 1800s.