Alex Marquez started competing in the CEV Buckler 125cc championship in 2010. He did not compete in the opening round of the season as he was not old enough, but he competed in the remainder of the season and finished 11th in the riders championship standings. He remained in the series for 2011 and battled for the championship with Alex Rins who ultimately took the title in the end. The series changed to Moto3 regulations for 2012 and with two race wins, Marquez took the title win. In 2012, Marquez made his world championship debut as a wildcard in Moto3. He made three wildcard appearances that year before replacing Simone Grotzkyj at Ambrogio Racing to finish the season full time. For 2013, Marquez moved to the Estrella Galicia 0,0 team where he achieved five podiums including a win in Japan. He remained with the team for 2014 and with ten podiums including three wins, he won the championship. He and older brother Marc became the first brothers to win championship titles the same year. 

For the 2015 season, Marquez made the move up to the Moto2 class with the Estrella Galicia 0,0 Marc VDS team where he finished 14th in the standings. He had a slightly better season in 2016 when he took his first Moto2 podium in Aragon. 2017 was a breakthrough year for Alex as he won his first Moto2 race and finished the year in 4th overall. He matched his championship position for the 2018 season despite not winning a race. In 2019, Marquez took ten podiums including five wins which won him the title, another double championship for the Marquez brothers. Marquez moved up to MotoGP to join his brother at the Repsol Honda Team replacing Jorge Lorenzo who had retired at the end of 2019. Alex took his first MotoGP podium in the wet in France and followed it up with a podium in his home race in France, making him the only Honda rider to achieve a podium finish in 2020. In 2021, Marquez moved to the LCR Honda Castrol team alongside Takaaki Nakagami. It was a fairly consistent season for the younger of the Marquez brothers as he finished inside the points in every race he finished. He remained with the LCR team for the 2022 season and finished in 17thplace overall. For the 2023 season, Marquez moves to the Gresini Racing MotoGP team alongside Fabio Di Giannantonio.