A former pupil of Coedcae Comprehensive School, Samson Lee joined the Scarlets at U16 level and came through the Academy system. Lee made 15 appearances for Llanelli RFC during the 2010-11 and 2011-12 seasons, scoring one try. He made his senior debut for the Scarlets in a pre-season friendly against Narbonne in August 2012 and went on to make 28 appearances for the region during his first senior season.
Lee was a member of the Wales U20 team during the U20 Six Nations Championship in 2012 and the IRB Junior World Championships in South Africa the same year, forming a strong partnership with fellow Scarlets front-row players Rob Evans and Kirby Myhill. Lee made his full Welsh debut, coming off the bench in the 2013 Autumn Series against Argentina. He was named in the 2013-14 RaboDirect PRO12 Dream Team.