Among his sabotage operations was the destruction of 25 Messerschmitt fighter aircraft and 150 engines stored in a bus garage in Oslo, on 14 August 1944, together with Max Manus, Gunnar Sønsteby and others.
[2] He cooperated with Kompani Linge soldier Max Manus in the SOE Operation Bundle, sabotage missions directed towards German ships.
Nielsen extracted information about the location from a contact in Oslo Sporveier, and also came along due to Gregers Gram's stay in Sweden at the time.
Nielsen kept guard in the night watch office; the other participants were Max Manus, Gunnar Sønsteby, Andreas Aubert, Henrik Hop, William Houlder and Erik Hansen Bakke.
There was a fire lasting three days in which the Luftwaffe lost 25 Messerschmitt Bf 109's, 150 aircraft engines and their largest store of spare parts for fighters in Norway.
He obtained a copy of the door key (via an imprint in plastilina) together with Dick Zeiner-Henriksen, but the actual sabotage mission was called off because there were people present at the office.
Nielsen went to the address Bygdøy allé 117, to what he assumed to be a safe house, but was surprised by German forces there and fatally shot while escaping.