In August 2017, he signed a professional contract for the following season with Vital Concept at the age of 24.[10] Mottier was one of two riders on the team from the Mayenne department, alongside Arnaud Courteille.[11] His first race with the team was the Grand Prix La Marseillaisein late January. In early March, he placed third in the Route Adélie after remaining in the breakaway for 200 kilometers. Two weeks later, he entered his first World Tour events, taking part in the Amstel Gold Race followed by the La Flèche Wallonne.
Mottier had a less successful second season as a professional, with his best result being 18th in the Circuit de Wallonie. Despite this, he remained with the team for a third season in 2020. After falling ill and suffering a fractured collarbone while training in December 2019, Mottier only competed in three races before the Covid-19 pandemic, including the Tour de Langkawi.[11][12] He returned to racing in August at the Mont Ventoux Dénivelé Challenge, followed by several more races throughout France. However, he lacked success once again and was this time not kept by his team.[13]
After he was not retained by B&B Hotels–Vital Concept, following three years with the team, Mottier announced that he was leaving professional cycling following the 2020 season.[14][15] In November 2020, it was announced that he would join French amateur team Laval Cyclisme 53, for the 2021 season.[3][16][17]
^ abGachet, Nicolas (12 November 2020). "Justin Mottier signe avec une N1" [Justin Mottier signs with an N1]. Directvelo (in French). Association Le Peloton. Retrieved 12 November 2020.
^Malvestio, Carlo (22 January 2019). "Presentazione Squadre 2019, Vital Concept – B&B Hotels" [Presentation of Teams 2019, Vital Concept - B & B Hotels]. SpazioCiclismo – Cyclingpro.net (in Italian). Gravatar. Retrieved 27 January 2019.