A graduate of École Nationale des Travaux Publics de l’État and École Nationale des Ponts et Chaussées, Thomas Allary began his career managing property projects in Morocco for the Ministry of Foreign Affairs, then moved on to road infrastructure project management at the Ministry of Transport. 

In 2008, he became Director of Transport for the Auvergne Regional Council before joining the cabinet of the Minister of State for Sustainable Development in 2010 as Transport Advisor. He joined Réseau Ferré de France in 2011, as project lead for the Paris–Orléans–Clermont–Lyon high-speed rail line. In 2012, he was appointed Regional Director for the Grand Est region at SNCF Réseau. In 2016, he joined TER as Deputy Regional Director for Auvergne-Rhône-Alpes, then returned to SNCF Réseau in 2017 as Regional Director for the same region. 

In 2023, he became Director of the SERM programme at SNCF Réseau and SNCF Gares & Connexions. In 2025, he was appointed Deputy Managing Director, Major Projects at SNCF Réseau.