Eight months later at the 2013 World Junior Championships in Dubai, Horton won five gold medals and a silver. He took out the 200 m, 400 m, 800 m and 1500 m freestyle events and alongside Luke Percy, Regan Leong and Blake Jones won the 4 × 100 m freestyle relay all in new Championships record times. The team of Horton, Leong, Isaac Jones and Jack McLoughlin finished second behind the British in the 4 × 200 m freestyle relay event.