2010-11: Brett Lernout played for the Warriors in the Winnipeg City Midget League. In 31 games he scored 3 goals with 12 assists and had 67 penalty minutes.
2011-12: Lernout attended the Saskatoon Blades pre-season camp and later appeared in two WHL games with the Blades a 16-year-old. He skated briefly for the Steinbach Pistons in the junior A Manitoba Junior Hockey League and was one of the top defensemen for the midget AAA Winnipeg Wild. Lernout was -2 with no points nor penalties skating for the Blades. He had one assist in six games with Steinbach. In 53 games for the Winnipeg Wild (including nine games in the MMHL playoffs), he scored 9 goals with 21 assists and 78 penalty minutes. Winnipeg won 41 of 44 games in the regular season and were unbeaten in the MMHL playoffs but lost to the Western Division champion Southwest Cougars in the 2012 Telus Cup West Regional.
2012-13: Lernout began the year with Saskatoon and skated in 18 games for the Blades before being acquired by Swift Current in a December trade. In 59 games between the two teams he scored 1 goal with 1 assist and was -4 with 28 penalty minutes as a WHL rookie. Swift Current finished third in the East Division and lost to Calgary in a first round playoff series. Lernout was -2 with no points nor penalties in five playoff games.
2013-14: Lernout skated in all 72 regular season games for Swift Current and led the Broncos with 103 penalty minutes in his second WHL season. He scored 8 goals with 14 assists and was +7. The Broncos finished second in the East Division and lost to Medicine Hat in six games in a best-of-seven playoff series. Lernout had 1 assist and was -1 in six playoff games. He was ranked 52nd amongst European skaters in Central Scouting’s final rankings prior to the 2014 NHL Draft.