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 became 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.
Wellens played for St Helens at fullback in their 2002 Super League Grand Final victory against Bradford Bulls but was forced to leave the field after just two minutes with a broken cheekbone, which ruled him out of Great Britain's intentional series against New Zealand the following month. With a point to prove, Wellens became one of Super League's most notable performers and made the number 1 jersey his own at international level. His attitude and aptitude to the game earned him a reputation as a model professional. As one of the quieter members of a squad littered with big names and personalities, it was all the more creditable that Wellens earned St Helens player of the season award in 2004 to underline his value at Knowsley Road, establishing himself as one of the fans' favourites. Wellens was selected in the Great Britain team to compete in the end of season 2004 Rugby League Tri-Nations tournament. In the final against Australia he played at fullback in the Lions' 44-4 loss.
In 2011, Wellens was named joint captain of St Helens alongside James Graham. He took over as sole captain following Graham's departure to Canterbury Bulldogs.
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 to win their first Super League title in 8 years, with Wellens lifting the trophy.