2009-10: Mason Geertsen was captain of the Leduc Oil Kings bantam AAA team in Alberta. He scored 5 goals with 17 assists and had 38 penalty minutes in 33 games. Geertsen was selected by Edmonton in the first round (18th overall) of the 2010 WHL Bantam Draft.
2010-11: Geertsen appeared in three WHL games with Edmonton, spending the bulk of the season with the Sherwood Park Kings midget AAA team in Alberta, and played for Team Alberta in the 2011 Canada Winter Games. He had no points and was -1 with 2 penalty minutes playing for the Oil Kings. In 31 games for Sherwood Park he scored 3 goals with 7 assists and had 84 penalty minutes. Sherwood Park reached the provincial semifinals against Geertsen's former club, Leduc. He scored 1 goal with 2 assist and had 24 penalty minutes in 10 playoff games. Geertsen scored 1 goal with 1 assist and had 4 penalty minutes in six games at the CWG as Team Alberta captured a bronze medal.
2011-12: Geertsen played in 34 games for the Edmonton Oil Kings in his first WHL season – suffering an injury on a huge hit in a January game against Swift Current and skating in just three games the rest of the year. He had 3 assists and was +5 with 70 penalty minutes. Edmonton captured the WHL title but Geertsen did not play in the playoffs or Memorial Cup. He played for Canada Pacific in the 2012 U17 World Hockey Challenge, finishing with no points and 2 penalty minutes in five games.
2012-13: Geertsen began the year with Edmonton before an October trade which sent him to the Vancouver Giants along with a draft pick in exchange for WHL veteran David Musil. He had 4 assists and was -2 with 32 penalty minutes in 15 games with the first-place Oil Kings and moved into a much larger role for the rebuilding Giants. He scored 2 goals with 8 assists and was -26 with 98 penalty minutes in 58 games for Vancouver. The Giants missed the WHL playoffs – finishing with the league's worst record. Geertsen was ranked 59th amongst North American skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.
Big, tough, rearguard. Already NHL-sized. Established early as a player prepared to take on WHL heavyweights. Able to control opponents in defensive zone. Should become more involved offensively as skating improves.