Johnsson was born in Pielisjärvi, and graduated from the Kuopion lyseon lukio (in Kuopio) on 5 June 1875. He obtained his law degree in 1879. Johnsson worked in the Vyborg Courts of Appeal from 1879, he served as reserve judge in 1882. Soisalon-Soininen worked at the senate's justice apartment from 1900 and from 1901 to 1905 as chancellor of justice. Soisalon-Soininen was assassinated in 1905 when a young student called Lennart Hohenthal murdered him at his apartment in Helsinki.
In the morning at 10.30 AM a young man of a heavy build (Hohenthal) dressed as a Russian officer came to Soisalon-Soininen's apartment in Helsinki. Having fooled the police posted outside, Hohenthal rang the doorbell and a valet (a disguised policeman) opened the door. Hohenthal gave the valet a business card which read Alexander De Gadd, Lieutenant de la Garde, St. Petersbourg. Hohenthal asked if he could meet the chancellor. The valet guided him to the chancellor's office.
When Soisalon-Soininen came to the room, Hohenthal pulled a pistol and shot eight shots towards the chancellor of justice, two of which struck the chancellor in the chest and stomach.
The chancellor of justice fell to the floor. Then the valet entered the room and shot at the assassin. During the exchange of fire between the assassin and policeman, the 17-year-old Johan (a.k.a. Juhani) who was the son of Soisalon-Soininen came to the room and joined the policeman in firing upon the assassin.
Hohenthal shot Johan in the leg. Hohenthal also received some small injuries and surrendered. He was taken to hospital under guard.