After being injured for most of the 2002 season, in 2003 he was awarded an AFL Rising Star nomination in his tenth game, after making his debut in Round 8. He was then injured again for most of the 2004 season with thumb and Achilles injuries.
Often criticised for poor performances early in his career, his performance as a key defender in 2005, including his exceptional performance in that year's Grand Final against the West Coast Eagles, Roberts-Thomson has secured a permanent position in the Swans' players list.
After an injury riddled 2008 season, Roberts-Thomson not only re-solidified his place at CHB in the Swans' defence in 2009, he improved vastly through the season playing on some of the best forwards in the AFL. .
In 2010, Roberts-Thomson continued his strong form and with Craig Bolton suffering a serious achilles injury early in the year, he and Grundy stepped up manfully to take on the brunt of the defensive work. Unfortunately, he injured his hamstring about halfway through the year and was forced to sit out the rest of the season.
2011 was an up and down year for Roberts-Thomson as injury and the rapid improvement from Ted Richards and Alex Johnson didn't allow for him to take up his normal position as a tall defender. Consequently, he was used mostly as a 2nd ruckman and tall forward when in the senior side. Illness to Grundy allowed LRT to take up a position in the backline late in the year however, a role that he would fill well for the Swans' run into the finals.
His father Barry has played in the Victorian Football Association for Sandringham and also captained the Queensland representative side. He became engaged to girlfriend Zoe Stenmark in 2011, they married at Shore Chapel in 2013.