At the 2012 Summer Olympics in London, Magnussen won a total of two medals, one silver and one bronze. In his first event, the 4×100-metre freestyle, Magnussen and his three other relay team members came fourth behind France, United States and Russia, despite being the favourites. As the lead-off leg, Magnussen recorded a time of 48.03, much slower than the 47.49 he led off in Shanghai. In the 100-metre freestyle final, Magnussen came in second by one-hundredth of a second to American swimmer Nathan Adrian 47.53 to 47.52, winning the silver medal. Overall, he said in an interview after his swim he swam his best but unfortunately it wasn't enough. In the 50-metre freestyle, Magnussen failed to advance to the final, finishing with a time of 22.00 in the semifinals. Issues pertaining to Magnussen's behaviour before and during the 2012 London Summer Olympics became apparent, due to reports that his Australian Swim Team teammates ultimately cheered upon his defeat in the 100-metre freestyle final. Magnussen was accused of harassing other teammates and having a "big head" during the games. There were also some complaints about the 100 metre relay team, dubbed the Weapons of Mass Destruction, who were called "out of control" by fellow swimmers.