Wellens joined the St. Helens ranks and made his Super League debut in the 1999 season. Success quickly followed and Wellens soon secured his place in perhaps St Helens strongest era. Wellens is renowned for his all round game, excelling particularly at taking catches. An England call up followed and with full back Kris Radlinski already established at international level, Wellens displayed his versatility with spells on the Wing and at Centre. A call up for Great Britain ensued, and he made a handful of appearances before being contentiously left out by then coach David Waite, who felt Wellens' form had dipped during this period. Wellens played for St Helens RLFC from the interchange bench in their 1999 Super League Grand Final victory over Bradford Bulls.
In 2011, Wellens was named joint captain alongside James Graham. Wellens is a Manchester City fan, and in May 2012 played in the match-up between St.Helens and Wigan in a heated local derby that Wigan won 42-16. Three players were sent off after a 26-man brawl at the Etihad Stadium as part of the Magic Weekend for 2012.
In 2014, Wellens made history by scoring the 1,000th point in his career. St. Helens reached the 2014 Super League Grand Final and Wellens captained them from fullback in what was a record-equalling 10th grand final appearance. Saints defeated Wigan Warriors 14-6.