At the 2013 European Cup Combined Events he topped the podium and set personal bests in the 100 metres (11.04 sec), long jump (7.63 m), shot put (14.95 m) and the discus throw (44.89 m).

[10] In 2014, at the European Athletics Championships held in Zurich, Mayer achieved a personal best of 8521 points in decathlon, winning him the silver medal.

[12] At the 2016 Rio Olympics, Mayer earned a silver medal with a new personal best of 8834 points, finishing behind only two-time gold medallist Ashton Eaton, who was the world record holder at the time.

[23] At the 2017 European Athletics Indoor Championships in Belgrade on 4–5 March, Mayer won the gold medal in the men's heptathlon.

[28] Mayer then took part in the decathlon in his adopted hometown of Montpellier on 13–14 May, and achieved a season best mark in discus and shot put.

[29] He also took part in the French Elite Outdoor Championships in Marseille on 14–15 July, but struggled in the rainy and windy conditions and finished the competition with a few no marks.

In a closely fought contest, Mayer narrowly beat Damian Warner of Canada by five points to win his first world indoor title.

He achieved his personal best in 100m and shot put and was leading the competition after 7 events, but withdrew before the pole vault due to a problem with his Achilles.

[42] At the 2021 European Athletics Indoor Championships held in Toruń, Poland, Mayer won the Men's heptathlon event with 6392 points.

[44] However, in the final day, he performed well in a few events, such as the hurdle and pole vault as well as a personal best in javelin throw, which allowed him rise in the ranking and win a silver behind Damian Warner of Canada.

[54][55] His paternal family and German surname have their origin in Farschviller, in the northeastern region of Lorraine,[a] where his father grew up; some of his relatives still live in the Moselle department next to the border with Germany.

[54] He has three brothers: Thibault, Thomas and Sébastien; the family was raised in La Roche-de-Glun, a small town by the Rhône river in the southeast of the country (Drôme department), where his parents still live.

[59] He studied for a Diplôme universitaire de technologie en mesures physiques (a technology-based degree in physical measurements, including metrology and instrumentation) at the University of Montpellier-Sète.