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 Maserati 4CLT/48 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.

[2] Mieres scored his best result of 1953, however, in the non-championship Grand Prix de l'Albigeois held in Albi, France, finishing in fourth place.

It is likely that an oil slick dropped by Mieres' Porsche caused a tragic accident at the 1958 Cuban Grand Prix, in which at least four people were killed and fifty more injured.