Stjernvall worked as manager in Finnish branch of Siemens & Halske 1904–1914 and subsequently in Kone ja Silta until 1929; the company grew during his time through new products and acquisitions.

[1][2][3] During 1904–1914, Stjernvall led technical office of Siemens & Halske in Helsinki;[1][2][3] in this period the company built the first steam turbine power plant of Finland in Tampere and connected Sveaborg to electrical grid through an undersea cable.

In 1918, Swedish People's Party of Finland asked him to join in Lauri Ingman's cabinet as Minister of Trade and Industry.

The articles of association were changed uniform with Kone ja Silta in December 1927, after which both companies shared the same management.

[1][2] Stjernvall handed over his managerial post of Hietalahti Shipyard and Engineering works to Robert Lavonius on 1 April 1929.