Zak Seddon first appeared in an international competition over 2000 m in 2009. At that time he started at the grammar school in Doha and finished in fourth place. In 2010, he qualified for the first Youth Olympic Games in Singapore in the same competition . With a personal best of 5: 52.13 min, he missed the finals, but came second in the B-final and ended up in tenth place overall. In 2011 he finished fourth over 1500 meters in the British U20 championships. At the national championships for adults, he started over the full obstacle distance of 3000 meters and came in sixth. In July he went back to the 2000 m obstacle course at the U18 World Championships 2011 in Lille, where he finished fifth with a new best time of 5: 40.62 minutes. Two months later he won the Junior Commonwealth Games in Douglas on the Isle of Man . Since 2012 he has been competing in the obstacle course over the full distance. He finished fourth at the British Championships in June. In July he competed at the U20 World Championships in Barcelona. He moved into the final, in which he was four seconds faster than in the preliminary run, then finished ninth with 8: 45.18 minutes.
The following year he started to compete more and more in the United States after studying economics at Florida State University in Tallahassee . He represented the university as part of its own sports team, the Florida State Seminoles. In April he improved his best time in the obstacle course to 8: 34.42 min in Palo Alto, California. After further competitions in the USA, he entered the U20 European Championships in Rieti in July , where he won the gold medal with a time of 8: 45.91 minutes. In 2014 he did not come close to his best performance. He only succeeded in doing this again in 2015. In July, he became British runner-up in the obstacle course. A year later he took third place at the national championships. Before that, in April he was able to improve his best time to 8: 33.09 minutes.
In March Seddon, who is trained by his father, set a new best time in Gainesville , Florida, in 8: 30.17 minutes. Then he also qualified for the World Championships in his British homeland. In the run-up he came to a time of 8: 32.84 minutes, which meant that he was eliminated in tenth place. Overall, his performance meant 22nd place. A month later he also performed at the Universiade in Taipei. In the final he ran 8: 39.30 minutes and came in fifth. After improving his best time in Germany by almost four seconds in May 2018, he became the British obstacle course champion for the first time in June. In August he finally started at the European Championships in Berlin, where he finished fifth. In 2019 Seddon set his current personal best at a competition in Rome, as of 2020, of 8: 21.28 min. In August he successfully defended his national championship title before taking part in the World Championships in Doha in October . He moved into the final as seventh of his preliminary run, in which he finished 15th and penultimate place with 8: 40.23 min.