The center forward started his career in the 1920s at the Club Français Paris; with this traditional club was Robert Mercier 1931 Cup Winners' Cup and in the same year to the national team. When in 1932 in France a professional league was introduced, he was with the Club Français doing, and quite successful: at the end of the first season of Division 1 won Mercier, the top scorer with Walter Kaiser von Stade Rennais UC, taking in a maximum of 18 games 15 results was achieved. His club, however, increased due to a decrease in the League from the second division in 1934 and withdrew entirely from the professional football back.
Mercier then moved to local rivals Racing Paris; in the 1935/36 season he was involved in his winning the double, was with the team champion of France (18 point games and eight goals ) and Cup Winners' Cup again, where he had been, however, not used in the final. In the following season he belonged to the tribe of the Elf, it even brought on 26 league games, but only scored one goal in it. A complicated fracture, which he drew upon in a friendly match with Racing Paris, ended his career abruptly. In that regard, it remains unclear whether he still came to inserts in the season 1937/38, when he should have been according to the website of the French Federation at Racing Arras under contract. The same applies to his subsequent employment at Stade Rennais UC and FC Dieppe ( as a coach ).
From May 1931 to November 1935 denied Robert Mercier seven A- international matches for France, where he scored three goals, two of them against England on his debut. But for the World Cup in Italy in 1934, he was not taken in France's squad because the competition was very strong attack on his position with Jean Nicolas and Roger Courtois.
- French Champion: 1936
- French Cup Winners: 1931, 1936 (excluding final use )
- 7 'A' matches, 3 matches
- League top scorer: 1933