He studied industrial design at the Istituto Europeo di Design in Milan and later pursued a master's in industrial design at the Domus Academy.

Guillaumier has collaborated with several well-known brands, including Minotti, Foscarini, Frigerio, Moroso, Driade, and Roda, and has developed a reputation for his refined, elegant, and functional approach to design. His work often balances craftsmanship with contemporary aesthetics, blending traditional influences with modern innovation.

In his partnership with Frigerio, Guillaumier introduced the Dao Soft modular sofa. This design emphasizes geometric harmony through soft, inviting volumes that offer extensive compositional freedom. The sofa's high backrests and seats are padded with varying densities of polyurethane foam and Cloud Foam, ensuring optimal comfort. Notably, the "a pizzicotto" stitching detail showcases Frigerio's commitment to craftsmanship. Additionally, Guillaumier designed the Pavos small tables for Frigerio. These tables feature curvaceous lines and are crafted from materials such as wood, glass, and concrete, reflecting a harmonious blend of form and function.

For Moroso, Guillaumier created the Josephine seating collection, inspired by the elegance and allure of Josephine Baker. The design is characterized by rounded, graceful forms where the armrests and backrest envelop the seat like petals of a flower. This collection includes sofas and the Josephine Vis à Vis, a tête-à-tête version that fosters intimate conversation. The elevated seat design imparts a sense of lightness, while the refined proportions exude sophistication.