During the 2012/2013 season, the first year in this category, Justine Braisaz was selected for the 2013 Junior World Championships in Obertilliach, Austria.

She ranked 12th in the individual race, 18th in the sprint, 14th in the pursuit, and 8th in the mixed relay with Chloé Chevalier, Dany Chavoutier, and Clément Dumont.

She earned her first podium in her World Cup career in Oberhof, Germany, by achieving second place in the women's relay on 7 January 2015 with her teammates; Marine Bolliet, Marie Dorin-Habert and Anaïs Bescond.

On 13 March 2015, at the 2015 World Championships in Kontiolahti, Finland, she won her first medal, silver, for the women's 4 x 6 km relay with Anaïs Bescond, Enora Latuillière and Marie Dorin-Habert.

At the French Biathlon Championships in La Féclaz, she won the relay title with Julia Simon and Marine Bolliet.

A week later, at the final round of the IBU World Cup season in Khanty-Mansiysk, Russia, she earned her first top 10 result with a 4th-place finish in the sprint event.

At the 2016 Biathlon Championships in Méribel, France, she earned the title of French Champion by winning the Mass Start even in the U21 category.