Um had his first international success on the short track at the 2009 Junior World Championships in Sherbrooke . There he took first places over 1500 m and the gold medal in the all-around in the 1500 m superfinal. In the 2010/11 season he was second with the relay at the World Cup in Dresden . He won the gold medal at the 2011 team world championships in Warsaw . At the end of January 2011 he won the silver medal over 1500 m and the gold medal with the relay at the Winter Asian Games in Astana and in March 2011 he won sixth place over 1500 m and fourth place with the relay at the 2011 Short Track World Championships in Sheffield . At the Winter Universiade 2013 in Trento , he won the silver medal in the 1000 m and 1500 m respectively. In speed skating, he started in Obihiro in the World Cup for the first time at the beginning of the 2018/19 season, finishing seventh in the team pursuit and third in the mass start. Before that he was South Korean champion over 5000 m in Seoul . At the following World Cup in Tomakomai he achieved second place in the mass start and in Heerenveen in the mass start he won his first World Cup victory and thus won the mass start World Cup at the end of the season. At the individual distance world championships in Inzell in 2019 , he won the silver medal in the mass start. He also finished seventh in the team pursuit there.
Personal bests
Personal bests in speed skating
500 m 37.22 sec. (installed on January 23, 2017 in Seoul)
1500 m 1: 48.78 min. (installed on September 24, 2017 in Calgary)
3000 m 3: 49.80 min. (installed on September 17, 2016 in Calgary)
5000 m 6: 21.48 min. (installed on September 22, 2017 in Calgary)
10000 m 13: 41.36 min. (installed on December 7, 2019 in Nur-Sultan)
Personal bests on the short track
500 m 41.472 sec. (installed on December 19, 2013 in Trento)
1000 m 1: 25.079 min. (installed on December 20, 2013 in Trento)
1500 m 2: 13.388 min. (installed on January 9, 2009 in Sheerbrooke)
Note: This article places the family name before the person's first name. This is the usual order in Korean. Um is the family name here, Cheon-ho is the first name.