During his early athletic career, Guamuch worked as a low-paid laborer in a textile factory.

On April 19, 1952, Guamuch participated in the Boston Marathon, a major international racing event.

He took the lead from his countryman Luis H. Velasquez after the first ten miles, and, against any predictions, finished ahead of U.S. competitor Victor Dyrgall by almost five minutes, recording a time of 2 hours, 31 minutes, and 53 seconds.

[2] The Boston achievement made Guamuch a sports icon in his country, and the Guatemalan government honored him by renaming the national stadium in Guatemala City to Estadio Nacional Mateo Flores.

He was also a golf caddie working at the Guatemala Country Club and occasional player.