Lavonius did his early career in German, Swiss, United States and Canadian companies, and Finnish Suomen Metalliteollisuuskonttori and Tampereen Pellava ja Rauta-teollisuus Oy.

As Lavonius could not get along with manager Wilhelm Wahlforss, he left the company and worked in arms industry until end of World War II.

Lavonius was appointed in 1927 deputy director of Hietalahti Shipyard and Engineering Works which Kone ja Silta had taken over in 1926.

Just few months after, in March 1929 Julius Stjernvall resigned for health reasons, and Lavonius became company manager of Kone ja Silta and Hietalahti yard in 1929 and Crichton-Vulcan in 1930.

Lavonius continued the company expansion, and in 1932 Kone ja Silta bought Kotka Engineering Works.

Subsequently, until the end of World War II his main activity was working in two state-owned arms industry companies as chairman of the board.