Born in Albany, Minnesota, Thelan played college golf at Foothills Junior College and Houston Baptist University where he was a teammate of Colin Montgomerie.

Won the PGA Club Professional Championship in 2000 and 2003.

Joined the Senior Tour after winning Qualifying School in 2010 and won his first over-50s title two years later - the Swiss Seniors Open - which he would win for the second time in 2016.