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 minus-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 plus-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 minus-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 minus-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 and was selected by Colorado in the fourth round (93rd overall) in the 2013 NHL Draft.
2013-14: Geertsen attended camp with the Avalanche before returning to Vancouver for his first full season with the Giants. Named an assistant captain, he played in 66 of 72 regular season games and was second on Vancouver with 126 penalty minutes while scoring 4 goals with 19 assists and finishing minus-5. The Giants were third in the B.C. Division and faced defending champion Portland in a first round series. Geertsen was scoreless with 14 penalty minutes, finishing minus-8, in the four game series.
2014-15: Geertsen signed a three-year entry-level contract with the Avalanche in March 2015 and made his pro debut with the AHL’s Lake Erie Monsters following his WHL season with the Vancouver Giants. He was -3 with no points and 2 penalty minutes in nine regular season games with Lake Erie. The Monsters finished fourth in the Midwest Division and lost to Manitoba in a seven-game playoff series but Geertsen did not see any action. Geertsen led all Giants’ defensemen in points (38) and penalty minutes (107). In 69 regular season games he scored 13 goals with 25 assists and was +3 as Vancouver finished last in the B.C. Division, missing the WHL playoffs.
Geertsen is a solid wall in his own zone at the junior level. If the puck goes into the corner in his end opposing players are unlikely to wrest it from him easily and if they do it will surely come with a price. He will come to the aid of teammates and is a willing combatant if a situation arises. Geertsen displayed some offensive elements to his game in his final two years of junior hockey.