As part of the Belgian 4 × 400 m relay team, he competed at the 2015 European Junior Championships and the 2016 World U20 Championships where they placed 5th and 11th respectively.
In 2017, he was eliminated in the first round of the 400 at the European U23 Championships having run a time of 48.52 and finished fifth with the relay in 3:06.45.
In 2019, he managed to reach the semi-finals of the European U23 Championships in Gävle but was ultimately disqualified.
In July 2022, Doom competed at the 2022 World Athletics Championships in the individual 400 m and 4 × 400 m relay. Individually, he made it through to the semi-finals before being eliminated running a time of 45.80 in the semi-finals. As part of Belgium's relay squad, Doom won a bronze medal, taking third behind the United States and Jamaica in a time of 2:58.72.
At the 2023 World Athletics Championships in Budapest, Doom again competed in the individual 400 m and 4 × 400 m relay. In the heats of the 400 m, he broke the 45 second barrier for the first time, running a personal best of 44.92 to qualify for the semi-finals. However, he was unable to advance to the finals after a 45.77 semi-final performance to take fifth in his heat.
Warholm had taken the lead at the break and held the lead until just 10–20 metres to go when he was edged out by Doom at the finish line. In winning gold, Doom also set an indoor personal best and Belgian national record in a time of 45.25, less than half a second off of his outdoor personal best.
The day after the 400 m finals, Doom competed alongside his Belgian teammates in the 4 × 400 m relay. In a similar manner to his individual 400 m victory the day prior, Doom, running the anchor leg, took the lead in the final meters to secure the win after having trailed for most of his leg, this time beating team USA's anchor Christopher Bailey at the line.
By taking both titles, he became the first male athlete to win the individual 400 metres and 4×400 metres relay at the same World Athletics Indoor Championships.
By taking both European titles, he became the first athlete to win the individual 400 metres and 4×400 metres relay at the World Athletics Indoor Championships and the individual 400 metres and 4 x 400 metres at the European Athletics Championships all four in the same year.
At the 2024 Summer Olympics in Paris, France, Doom, having skipped the heats, started off the Belgian team in the final that finished 4th in the Mixed 4 × 400 metres relay.[9] Having qualified for the men's 400 metres semi-final, Doom, suffering from pubalgia, was forced to pull up in the last bend and did not finish the race.[10] It was his last race at the Paris Olympics.[11]
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