2010 At the
2010 Australian Swimming Championships, Magnussen came third in the 100-metre freestyle, with a time of 49.43, thus qualifying for the national team. In the 2010 Pan Pacific Championships, Magnussen earned a silver medal in the 4x100 metre freestyle relay, with
Eamon Sullivan,
Kyle Richardson, and
Cameron Prosser in a time of 3:14.30, finishing behind the United States. Swimming the anchor leg, he had a split of 48.25. In the 100-metre freestyle, Magnussen finished 10th overall with a time of 48.94. At the
2010 Commonwealth Games in Delhi, Magnussen won a gold medal in the 4×100-metre freestyle relay, with Kyle Richardson, Eamon Sullivan, and
Tommaso D'Orsogna in a time of 3:13.92. At year's end, Magnussen competed at the
2010 FINA World Swimming Championships (25 m) in Dubai, and was part of the Australian team that finished 5th in the 4×100-metre freestyle relay.
2011 World Championships At the
2011 Australian Swimming Championships, Magnussen qualified for the national team by winning the 100-metre freestyle in a time of 48.29. At the
2011 World Aquatics Championships, Magnussen won a total of three medals, two golds and one silver. In his first event, the 4×100-metre freestyle relay, Magnussen won gold with
Matthew Targett,
Matthew Abood, and
Eamon Sullivan in a time of 3:11.00. As the lead-off swimmer, he had a time of 47.49. Magnussen's lead-off time eclipsed
Pieter van den Hoogenband's (unofficial) pre-polyurethane suit record of 47.84, and, at the time, was the fastest swim ever over that distance by a swimmer not wearing the now-banned suit. Four days later, Magnussen became the first Australian man to win the 100-metre freestyle at a World Championships, with a time of 47.63 seconds. At the first 50 in the 100-metre freestyle, Magnussen was in fifth place in 22.94, but came back fastest in 24.69 for the win. After the race, Magnussen said, "When I get back to Australia, I will be relaxing with my friends, and it will sound amazing to be called a world champion. It has been six weeks since I have gotten a good night's sleep. No Australian has won this race at the world championships before, so it is good to be in the same club as the legends of this sport." On the final night of competition, Magnussen won a silver medal in the 4x100-metre medley relay with,
Hayden Stoeckel,
Brenton Rickard, and
Geoff Huegill in a time of 3:32.26. Swimming the freestyle leg, Magnussen had a split of 47.00, by far the fastest in the field and was almost enough to overtake the gold winning Americans for 1st place.
2012 Australian Swimming Championships At the
2012 Australian Swimming Championships, which also served as Australia's Olympic trials, Magnussen qualified for the
2012 Summer Olympics, by becoming the national champion in the 50 and 100-metre freestyle. In his first event, the 100-metre freestyle, Magnussen recorded a time of 47.10 to win by over half a second. His 100-metre freestyle time was the 4th fastest swim in the history of the event, and also lowered his own unofficial textile record of 47.49. In the 100-metre freestyle final, Magnussen had a drastic change of tactics compared to his swims in Shanghai. At the first 50, Magnussen had a swift split of 22.68, and came back in 24.42. In the 100-metre freestyle final in Shanghai, he went out in 22.94 and came back in 24.69. Thus, Magnussen was more aggressive in the first 50-metres. Magnussen has stated he does plan on targeting
César Cielo's
world record of 46.91 in the 100-metre freestyle. After topping the heats (22.19) and semifinals (21.87) in the 50-metre freestyle, Magnussen won the final in a time of 21.52, the fastest any Australian has ever been in textile swimwear in that event. Prior to this competition, Magnussen's personal best in the 50-metre freestyle was 22.41.
2012 Summer Olympics At the 2012 Summer Olympics in London, Magnussen won 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 having a "big head" during the games, with claims that "he did not feel the need to attend team meetings and that he did not want to sit poolside to watch other Australian swimmers compete". There were also some complaints about Magnussen and his 100 metre relay teammates, dubbed the Weapons of Mass Destruction, who were accused of harassing other Australian swimmers who in return described the quartet as "out of control". He also competed in the 50 m freestyle event, but was unable to make the final, finishing ninth in the semifinals. He finished off the competition by anchoring Australia's
4x100-metre medley relay, with them going on to win the silver medal, A lower back injury disrupted his 2014 season, and he also had a shoulder injury which required surgery in 2015. However, in May 2025, Enhanced Games announced that Greek swimmer
Kristian Gkolomeev had unofficially broken the long course
50 metres freestyle world record at an February 2025 private event, beating Magnussen to the record, and winning the $1 million prize. He reportedly swam 20.89 seconds, 0.02 seconds faster than the current record by
César Cielo. ==Personal bests==