Fajdek represented Poland at the 2008 World Junior Championships in Bydgoszcz, finishing just off the podium in 4th despite throwing a new Polish junior record of 75.31 metres with the 6 kg under-20 implement.[2] In 2009, his first year as a senior, Fajdek placed 8th at the European U23 Championships in Kaunas, Lithuania with a throw of 68.70 metres.[3] Fajdek placed 3rd at the Polish Championships that year with a mark of 70.86, behind former Olympic and World Champion Szymon Ziółkowski and a returning Wojciech Kondratowicz.[4] In 2010 he grabbed another national bronze, this time with a throw of 73.85 metres; later that year, he improved his personal best to 76.07, good enough for 35th in the world.[5][6]
Fajdek first broke 80 metres at the OstravaGolden Spike meeting in May 2012, finishing second to Hungary's World silver medallist Krisztián Pars.[12][13] In his next competition at Montreuil he threw yet another personal best of 81.39, this time beating Pars.[14]
In 2022, by winning gold medal at the 2022 World Athletics Championships with the result of 81.98 meters, Fajdek became only the second athlete in history to win five back-to-back gold medals at the event, after pole vaulter Sergey Bubka, who eventually won six consecutive titles.[17]