Mieres competed in a number of different sports, including rowing, sailing, rugby and tennis. He first took up motorsport in Argentina with an MG, which he later replaced with a Mercedes-Benz SSK and then a Bugatti formerly driven by Achille Varzi, using the latter to win the Argentine sports car championship. As a reward, he was invited to join his compatriots Juan Manuel Fangio and José Froilán González on a trip to Europe, during which he finished fourth in the 1950 Circuit des Nations in Geneva driving a &lt;a href="Maserati%204CLT/48"&gt;Maserati 4CLT/48&lt;/a&gt; Formula Two car. After returning to Argentina, he was recruited by the Gordini team to replace the injured Jean Behra, allowing him to make his début in the World Championship at the 1953 Dutch Grand Prix. He also competed in the French and Italian Grands Prix, taking a best finish of sixth at Monza.