2007-08: Borna Rendulic moved from Croatia to western Finland and skated for the S-Kiekko club in Seinajoki — playing for the lower division clubs U18 and U16 teams — and skated for Croatia's U20 and U18 teams in their respective World Junior Championship tournaments. Rendulic scored 26 goals with 11 assists and 12 penalty minutes in 14 games for S-Kiekko. He was fifth in scoring amongst all skaters for Croatia at the 2008 Division 2 Group B U20 World Junior Championship in Estonia. In five games he scored 7 goals with 3 assists and was +3 with 4 penalty minutes for third-place Croatia. Rendulic led Croatia's U18 team in scoring at the 2008 Division 2 Group A tournament in France. He scored 7 goals with 5 assists and was +7 with 2 penalty minutes in five games. Croatia was third behind France And Korea in the tournament.
2008-09: Rendulic joined Assat Pori — skating for the club's U18 team in Finland's top junior hockey division as a 16-year-old — and played for Croatia's national team at the men's level and in U20 and U18 championship play. He scored 13 goals with 13 assists and 18 penalty minutes in 31 games for Assat Pori. Rendulic scored 1 goal with 2 assists and was -4 with 2 penalty minutes as Croatia's men's team finished fifth in the six-team Division 1 Group A tournament in Lithuania. He had 3 assists with 4 penalty minutes and was +5 in four games for the U20 squad that finished first in the Division 2 Group B tournament in Spain and scored 3 goals with 3 assists and was +1 with 2 penalty minutes in five games as the Croatia U18 team finished fourth in the Division 2 Group A tournament in nearby Slovenia.
2009-10: Rendulic played nine games, including two playoff contests, with Assat Pori's U20 team and spent most of the season with the club's U18 squad. He represented Croatia in the 2010 Division 1 Group B World Championship and 2010 U20 Division 1 tournaments. Rendulic scored 2 goals with 2 assists and was minus-1 with 4 penalty minutes playing for Assat's U20 team. He scored 12 goals with 19 assists and had 51 penalty minutes in 29 regular season games for the club's U18 squad. In nine playoff games he scored 3 goals with 5 assists as Assat Pori won a bronze medal. Rendulic had 1 assist and was -5 in five games as Croatia's men's team failed to win a game and was relegated to Division 2 for 2010-11. Croatia finished fifth out of six teams in the U20 Division 1 Group B tournament and Rendulic was the tournament's eighth-leading scorer with 4 goals and 3 assists, finishing -4, in five games.
2010-11: Rendulic played 31 games for the Assat Pori U20 team and returned home to Zagreb to skate for the Medvescak men's club in the Austrian pro league; once again playing for the Croatia national team at the men's and U20 levels. In 17 games for Medvescak Zagreb's top team, including five playoff games, he scored 2 goals with 2 assists and was +1 with 8 penalty minutes. He played for Zagreb's second team in the Croatia league playoffs and scored 1 goal with 5 assists in four games. Rendulic scored 4 goals with 14 assists and had 36 penalty minutes in 31 junior games with Assat. He dominated the lower division competition skating for Croatia in the 2011 Division 2 World Championship tournament in his hometown, finishing one point behind teammate Marko Lovrencic for the overall scoring lead with 17 points in five games. Rendulic scored 8 goals with 9 assists and was +10 with 6 penalty minutes as Croatia finished second to Romania. Croatia's U20 team was fifth in the 2011 Division 1 Group B tournament in Slovenia. Rendulic led his team and was the tournament's fifth-leading scorer with 4 goals and 5 assists in five games.
2011-12: Rendulic made his SM-Liiga debut — appearing in 3 games with Assat Pori — and also played for SaPKo in Finland's second division; leading the Assat U20 team with 61 points in 39 games (including playoffs). He had no points and was minus-2 in limited ice time with the Assat men's team. Rendulic scored 2 goals with 3 assists and was -1 with 14 penalty minutes in seven Mestis games with SaPKo. He scored 21 goals with 28 assists and was +20 with 41 penalty minutes in 34 regular season A Junior games and led Assat Pori with 4 goals, 8 assists and 27 penalty minutes in five playoff contests. Rendulic scored 4 goals with 1 assist and was -2 in five games as Croatia finished third in the 2012 World Championship Division 2 Group A tournament in Iceland. He played for Croatia's U20 team at the 2012 Division 1 Group B Junior World Championship in Poland and was the tournament's fourth-leading scorer with 3 goals and 4 assists; finishing -2 in five games as Croatia finished fifth.
2012-13: Rendulic played 37 regular season SM-Liiga games and 5 playoff contests with HPK Hameenlinna after being signed by the club over the off-season. He also spent time in Mestis with Peliitat Heinola and skated for Croatia's national team in the 2014 Winter Olympic qualifying games and in the 2013 Division 2 Group A World Championship tournament. He scored 8 goals with 4 assists and was +4 with 6 penalty minutes during the regular season and was minus-1 with 1 assist in the playoffs for HPK. In five Metsis games for Peliitat he scored 2 goals with 2 assists and was -2. Rendulic scored 1 goal with 1 assist and was -5 with 14 penalty minutes in three Olympic qualifying games. Croatia won a gold medal as the host team of the D2A WJC tournament and Rendulic was +6 with 4 goals and 4 assists in five games as an assistant captain.
2013-14: Rendulic led HPK Hameenlinna in scoring in his second season and was an assistant captain for Croatia at the 2014 Division 1 Group B World Championship. He scored 11 goals with 21 assists and was minus-1 with 34 penalty minutes in 57 regular season games. HPK finished tenth in the re-christened Liiga, securing the final playoff spot, and swept Jokerit in the preliminary round before falling to Kanada Cup champion Karpat. Rendulic scored 3 goals and was minus-1 with 2 penalty minutes in six playoff games. Croatia finished second to Poland in the 2014 D1B WC and Rendulic scored 2 goals with 2 assists and was +2 with 2 penalty minutes in five games. He signed a two-year entry-level contract with the Colorado Avalanche on May 19th, 2014.
Rendulic signed an entry-level contract with the Colorado Avalanche on May 19th, 2014