From 1934 to 1939 he was the assistant officer (Estonian: käsundusohvitser) of the state elder Konstantin Päts.

[1] In 1939, the then 59-year-old Jonson resigned from active service with the rank of Major General due to exceeding the age limit.

After the Stalinist Soviet invasion and occupation of Estonia on 16-17 June 1940, a few days later Jonson was reinstated to service and appointed Commander of the Estonian Army to replace the previous Commander-in-Chief Johan Laidoner who had been forced to resign by the Soviet authorities.

[1] Jonson formally remained commander of the armed formation after it was renamed the "Soviet Estonian People's Army", and then also after it became part of the Soviet Red Army' (as the "22nd Estonian Territorial Rifle Corps").

On 13 June 1941, Jonson was sent to the General Staff Academy in Moscow, where the Soviet authorities arrested him on 19 July 1941.