He moved to Buenos Aires in 1938 and began training in San Lorenzo de Almagro under Francisco Mura. At the same year, he won his first national championships title (in 5000 m). Over the years he would win 9 more titles, but none of them in marathon.
The LondonOlympic Games were Cabrera's first major international tournament. Etienne Gailly from Belgium dominated the marathon race most of the way and until almost the very last moments. He was the first to enter the stadium, but with 400 m to go, Gailly fell down, exhausted, got up and fell again. It was like Dorando Pietri's dramatic finish 40 years before. Cabrera and Tom Richards of Great Britain managed to pass the staggering Gailly, Cabrera winning by 0.56 seconds ahead of Richards.