2010-11: Darby Llewellyn skated for the Belle Tire '96 team in Michigan. He was invited to USA Hockey's Select 15 Player Development Camp in Rochester.
2011-12: Llewellyn played for the Honeybaked U16 midget minor team. In 28 games he scored 13 goals with 13 assists and had 34 penalty minutes. Llewellyn was selected by Kitchener in the fourth round (71st overall) of the 2012 OHL Priority Draft.
2012-13: Llewellyn appeared in 40 regular season games and one playoff contest for Kitchener in his first OHL season — missing time with a concussion. He scored 7 goals with 6 assists and was +1 with 15 penalty minutes in 41 OHL games.
2013-14: Llewelyn returned to Kitchener for his second OHL season. Skating in 66 of 68 games for the Rangers he scored 25 goals, sharing the team lead with Justin Bailey (BUF), and was -13 with 11 assists and 43 penalty minutes. Kitchener missed the OHL playoffs; finishing last in the Midwest Division. Llewellyn was ranked 113th amongst North American skaters in Central Scouting's mid-term rankings prior to the 2014 NHL Draft.