Born in Uddeholm in Värmland, Sweden on October 10, 1730, to Bengt Gustaf Geijer and Lovisa Sophia Tranæa, Geijer married Sara Helena Piscator in 1753 and had children.

[1] Geijerstam served as a director at the Uddeholm Works, a privately held company, from 1753 to 1767, which later developed into the multinational steel producer Uddeholms AB.

In 1770, he obtained the Bofors Works from his brother Johan Eberhard Geijer.

[2] Geijerstam lived in Alkvettern Manor located in Bjurtjärn socken.

He received the Royal Order of Vasa and was ennobled in 1773 at Loka Brunn by Gustav III,[1] thus the progenitor of the noble family af Geijerstam.