Axelsen was born in Odense. In October 2010 he claimed his first international senior title at the age of just sixteen, winning the Cyprus International. A few weeks later he entered his first Super Series event in singles, the Denmark Open 2010; making it through the qualifying stages before losing out to compatriot and eventual winner Jan Ø. Jørgensen in the second round. He won the 2010 BWF World Junior Championships in Guadalajara, Mexico.
Axelsen secured gold at the European Junior Championships, defeating teammate Rasmus Fladberg 21–8 17–21 21–13 in the final. He took a silver medal at the 2011 BWF World Junior Championships.
Viktor Axelsen finished runner up at the French Open in Paris, losing in the final to Daren Liew 18–21, 17–21. He also won a bronze medal at the 2012 European Badminton Championships losing the semi-final in three games to Sweden's Henri Hurskainen 21–18, 18–21, 17–21.
Axelsen has won the Swiss Open Grand Prix Gold in early month of 2014 beating China's Tian Houwei in the final 21–7, 16–21, 25–23. Axelsen won a bronze medal at the 2014 BWF World Championships and also a bronze medal again at the 2014 European Badminton Championships.
Axelsen finished runner-up at the Swiss Open Grand Prix Gold losing in the final to India's Srikanth Kidambi 15–21, 21–12, 14–21.
Axelsen finished runner-up at the Australian Open Super Series losing to Chen Long of China in the final 12–21, 21–14, 18–21.
Axelsen finished runner-up at the Japan Open Super Series losing to Lin Dan of China in the final 19–21, 21–16, 19–21.
Axelsen earned his first European crown in May 2016 beating compatriot and defending champion Jan Ø. Jørgensen with 21–11, 21–16 in the final of the 25th edition of the European Championships, the first in France at La Roche sur Yon. Viktor Axelsen was also part of the historic Danish team winning the first ever Thomas Cup title in 2016. Axelsen won five of his six played singles matches in the team tournament, also against Indonesia's experienced player Tommy Sugiarto in the final (21–17, 21–18) setting up a dramatic and historic 3–2 victory for Denmark over Indonesia. In the 2016 Rio Olympics, Viktor won the bronze medal by beating Lin Dan from China 21–15, 12–21, 21–17. Misbun Sidek was his personal trainer 6 months before the Olympics in Rio.
Axelsen won the World Championship in Glasgow in two sets against Lin Dan (22:20, 21:16) and became the third Danish Player to ever become a world champion (Peter Rasmussen 1997 in Glasgow & Flemming Delfs 1977 in Sweden). Axelsen, with a record of 4-3, is the only top twenty player to hold a winning record against Lin Dan, head-to-head.Key
In addition to his native Danish, Axelsen is also a fluent speaker of English and Mandarin.