Phi Delta Gamma originated from a high school fraternity, Alpha Iota Omega, which was founded in 1939. When the brothers of Alpha Iota Omega enrolled in the College of Agriculture and Mechanic Arts (Colegio de Agricultura y Artes Mecánicas) of Mayagüez, some were offered the opportunity to pledge one of the original Puerto Rican fraternities. The others reunited in the house of the brothers Gilberto Oliver Padilla and Otis Oliver Padilla. They decided to found a new fraternity on April 25, 1942, and to use the Greek letters Phi, Delta and Gamma.
The founding brothers are Gilberto Oliver Padilla, Otis Oliver Padilla, Raúl Romaguera, Ramón Antonio Frontera, Ignacio Beauchamp, José Ramón Ponce de León, Hilton Quintana, Angel Inserni, Rafael Ferrer Monge “El Cubano” (who also developed the shield, the first rulebook, and the motto of the fraternity), Rubén D’Acosta, Edgardo Olivencia, César Arana and Hernán Rodríguez.
These brothers established the first chapter as Alpha in Mayagüez. They had plans to expand the fraternity to all of Puerto Rico. In 1944, the first Phi Delta Gamma clubhouse was established in the Méndez Vigo street of Mayagüez.
In 1945, brother Otis Oliver commended to students Juan Mari Brás and Manuel Portela Lomba to take the Phi Delta Gamma fraternity to San Juan. They began the task of establishing the Beta chapter in the University of Puerto Rico in Río Piedras. Some of the founding brothers of the Beta chapter are Oscar Colón, Juan Mari Brás, Jean García, Ramón Vargas, Manuel Portela Lomba and Angel Amézaga. Thanks to this effort, the first convention of the Phi Delta Gamma was celebrated in 1945, in the Condado Hotel.
The Phi Delta Gamma members are known as "the originals", because they changed the way a traditional fraternity may be; also, they were the first Greek letter fraternity, which originated in western Puerto Rico: town of Mayagüez. The Phi Deltas are renowned for their friendliness and honesty. Above all, the great brotherhood among its members is very characteristic of their family activities (which involve grandparents, parents, spouses, children, grandchildren, and even great-grandchildren).