Sønsteby spearheaded some of the most importation sabotage missions carried out by resistance fighters during the Nazi occupation of Norway in World War II, targeting train stations, industrial centres and fuel stores.
The one-time head of a highly effective sabotage unit known as the Oslo Gang, Sønsteby was also known as Kjakan, Erling Fjeld and Nr. 24.
Sønsteby is the only Norwegian ever to have received the War Cross with three swords.
In April he also became the first person to receive the Norwegian Defence Cross of Honour.