[3] On the maternal side he was a nephew of Ole Rømer Aagaard Sandberg, and thus a grandnephew of Ole Rømer Aagaard Sandberg, Sr.[4][5] On the paternal side he was a second great grandson of Carl Rønneberg,[4] and a grandnephew of politician Anton Johan Rønneberg, whose mother was a part of the Holmboe family—hence Joachim's middle name.

[4][7] He last lived in Ålesund[8] where a statue honouring him made by Håkon Anton Fagerås was unveiled by Princess Astrid at the end of August 2014.

[12] After landing at a location 45 kilometres (28 mi) from the other team Gunnerside spent five days waiting out an intense blizzard in an uninhabited hunting cabin before meeting up with Grouse.

[14] The combined Norwegian team went into action against the Norsk Hydro heavy water production plant in Vemork in 1943, parachuting into the Hardangervidda plateau on 16 February.

[11] Rønneberg led the demolition team when the saboteurs, on the night of 27–28 February 1943, entered the Norsk Hydro plant and set explosive charges.

The team then escaped from the factory as the explosives went off, without the German guards discovering the saboteurs or indeed noticing that there had been an attack on the plant, probably believing that the heavy snow had set off one of their own land mines.

"[12] Although chased by 2,800 German troops,[15] five of the saboteurs, led by Rønneberg, escaped safely to neutral Sweden by way of a 14-day march over a distance of 400 kilometres (250 mi)[16] after the successful completion of their mission.

[15] After the factory was reported to have been rebuilt in the summer of 1943 a new saboteur attack was planned, but eventually scrapped in favour of an air strike.

[22] In 1965 the less-than-accurate American film The Heroes of Telemark, starring Kirk Douglas and Richard Harris, was released by Columbia Pictures.

[11] In 1995, Rønneberg, together with fellow World War II resistance leader Gunnar Sønsteby and Norwegian businessman Erling Lorentzen, received the Norwegian-American Chamber of Commerce Achievement Award for "individuals whose outstanding personal accomplishments exemplify the spirit of commitment, perseverance and endeavor that sustains the strong relations between Norway and the United States of America".

[35] In April 2013, Rønneberg was presented with a Union Jack during a ceremony at the Special Operations Executive (SOE) monument in London to mark 70 years since the successful Gunnerside mission.