Born in Clermont-Ferrand, Puy-de-Dôme, Pladner won a gold medal at the 1925 European Amateur Boxing Championships, and made his professional debut in January 1926 with a win over Rene Boriello. He won his first 13 fights before being held to a draw in January 1927 by Kid Socks. Awarded the French flyweight title after opponent Francois Moracchini withdrew at the last minute, he defended it successfully against Moracchini in February 1927, and over the course of that year added wins over Michel Montreuil, Alf Barber, Nicolas Petit-Biquet, and two further wins over Moracchini. He suffered the first defeat of his career in December 1927 when he lost a points decision to Johnny Hill at the National Sporting Club in London.