According to the group, during Hieronimus’s 30-year career at L'Oréal, he is credited for having developed the skin care business, with the creation of Dermo Expertise, Solar Expertise and Men Expert.
According to L’Oréal’s website, Hieronimus became marketing director for the Laboratoires Garnier in 1993, and created the Fructis haircare range. In 1998, he became General Manager of the Garnier Maybelline division in the UK. In 2000, he was named General Manager, L'Oréal Paris France, and then International General Manager for L'Oréal Paris, which he repositioned as the "accessible luxury" brand. In particular he developed skincare with the creation of Dermo expertise, Solar expertise, and Men expert. In 2005, he became General Manager of L'Oréal Mexico. In 2008, he became General Manager of the L’Oréal Professional Products Division and he increased its global leadership, driven in particular by the launch of Inoa hair color. In January 2011, he was appointed President L’Oréal Luxe. In addition to this role, he took up his new position as President Selective Divisions (Luxury, Active Cosmetics, Professional Products, The Body Shop) in July 2013.