Willis finished 4th in the 2013 World Championships and in the 2013 European Championships (SC) (both in the 200-metre breaststroke). He finished 4th in the 2014 European Championships and went on to win a bronze medal at the 2014 Commonwealth Games in the 200-metre breaststroke final in Glasgow. He finished 4th in the 2015 World Championships and won a bronze medal in the 2015 European Championships (SC) in the 200-metre breaststroke. He participated in the 2016 European Championships, and finished 4th in the final of the 200-metre breaststroke in the 2016 Summer Olympics in Rio. He went on to win a silver medal in the 2016 World Championships (SC). Willis made the final for both the 100-metre and 200-metre breaststroke in the 2018 Commonwealth Games, where he finished 8th and 4th respectively. In July 2018, Willis retired from competitive swimming.