Seto took up swimming at the age of five. He narrowly missed the 2012 Olympics when he finished third in both the 200 m and 400 m individual medley events at the national selection. He improved at the 2012 FINA World Cup, where he competed in all stages and ended up fourth overall. He concluded the short course season at the 2012 Short Course World Championships. Here Seto won the first international medals of his career. He first competed in the 400 m individual medley, where he won his first World Championship title in a new Asian record of 3:59.12, over a second and a half faster than second-place finisher László Cseh. He then competed in the 200 m individual medley, where he was able to rally past Cseh for the silver in a time of 1:52.80, over three seconds behind winner Ryan Lochte, who broke the world record.