He was born at Villnäs in Finland and began his military career in the army, as a cornet in the Field Marshal's Guards Company.

During this period the dockyard was not under direct Crown control but was leased by private entrepreneurs, Henrik Hybertsson and Arendt de Groote.

He was instrumental in organizing the expedition to establish the Swedish colony in North America, New Sweden, in modern Delaware, in 1637.

He was also an industrialist, and in the 1630s established an ironworks at Vira, which produced weaponry for the Swedish armies in the Thirty Years War.

[2] Clas was born to Lars Hermansson Fleming and Anna Henriksdotter Horn af Kanckas.