Maillard started playing hockey with Yverdonjunior teams and then moved to Lausanne to join their U15 team in 2011. He went on to play 3 full seasons with the U15 and U17 teams before following his brother in joining HC Fribourg-Gottéron U17 and U20 teams for the 2014-15 season. With little hope of joining Fribourg's professional team in the future, Maillard joined Genève-Servette HC U20 team for the 2015-16 season. He played 24 games for the team this year, putting up 17 points and added 5 points in 3 playoffs contests.
Genève-Servette HC
2016-17
Maillard made his professional debut with Genève-Servette HC in the 2016–17 season, appearing in 8 National League games this season. He spent the majority of the season with Geneva's junior team in the Elite Junior A, where he played 37 games and put up 51 points.
2017-18
Maillard eventually played one more year with Geneva's junior team in the 2017–18 season and helped the team win the Elite Junior A championship, tallying 8 points in 9 playoffs games. He scored his first NL goal with Genève-Servette that same year. Maillard also had a brief stint in the Swiss League, appearing in 5 games with HC Ajoie.
2018-19
On May 31, 2018, Maillard signed a two-year contract with Genève-Servette, ending his junior career. Prior to the start of the season, he switched his jersey number from 98 to 11. Maillard was invited to the New York Islanders prospect camp in the summer of 2018. Maillard missed the start of the 2018–19 season with a lower body injury before being assigned on a rehab stint with HC Sierre of the MySports League. After having played 12 games with Sierre, putting up 10 points, Maillard returned to Geneva and made his NL season debut on November 20at home, against HC Fribourg-Gottéron.
2019-20
On September 30, 2019, Maillard agreed to an early two-year contract extension with Geneva through the 2021/22 season. Maillard struggled to produce offensively during the 2019–20 season and was sent on a loan to HC Sierre of the Swiss League but only appeared in one game, scoring one goal, before being called up by Geneva. He finished the season with 8 points over 44 regular season games.
