He received a private schooling.
Before going to Costa Rica, he single-handedly started the cattle industry on Padre Island, on the Texas coastline. Simultaneously, his older brother Henry had won a contract to build a railway from Costa Rica's populated central highlands to the unpopulated and relatively unknown Atlantic coast. Minor joined him in Costa Rica to help with the project. Arriving in mid-1871, he was sent to the Atlantic coast hamlet of Port Limón to start building from that end, while Henry started work on the inland section.
On August 10, 1871, Henry started in the highlands, and on November 15 Minor started from Port Limón. By November 30, 1873, the first 27 miles of relatively easy highland construction was completed. For various reasons Henry was forced to halt construction at this point. Because he was unable to meet the terms of his contract, it was canceled by the Costa Rican government. Minor, by then established as a businessman and entrepreneur in Port Limón, negotiated a new contract with the Costa Rican government and in 1875 continued work on the lowland portion of the railroad. Nevertheless, there were several periods of relative inactivity in construction because of the extremely difficult terrain and the meager financial resources of Costa Rica, which then had but 146, 000 inhabitants and no industry. Further problems included diseases from the swampy jungles (it is said 4, 000 workers died building the railroad) and the difficulty of obtaining labor. To solve the labor problem, Keith imported many West Indians, especially Jamaicans, whose decendants to this day live in the Port Limón area, many still speaking West Indian-accented English. Another source of labor was Ferdinand de Lessep's failing Panama Canal project which released many workers.
In the early 1880s, to arrange financing for the last stretch between the completed high- and lowland sections of railroad, Keith negotiated a series of contracts, refinanced earlier loans, and on August 20, 1886, began to build the final portion. This was completed 4 years later, and on December 7, 1890, the first train covered the 97 miles of narrow-gage line to San José, Costa Rica's inland capital city, from Port Limón.
Simultaneously with the railroad construction, in 1884 Keith shrewdly obtained 800, 000 acres of uncultivated land adjacent to the railroad right-of-way. On this land he planted the succulent Gros Michel strain of banana, which he had imported from Panama. Earlier, Keith had experimented with a small shipment to the United States and found them popular. Soon with his own crops, railroads to transport them, and his own port and vessels, with volume exports to the United States, Keith created a huge domestic American market. In order to stabilize this market, in 1899 he and several other banana importers organized several smaller companies into the pioneer banana giant, the United Fruit Company.
Though he is primarily associated with Costa Rica, Keith also developed extensive banana and railroad interests in Guatemala, El Salvador, Nicaragua, Panama, Colombia, Venezuela, Brazil, Jamaica, and the Dominican Republic. In fact, the very success of his banana activities often tided him over the financial shoals of his 20-year Costa Rican railroad-building enterprise.
In 1883 Keith married into one the most aristocratic Costa Rican families. As the 19th century drew to a close, he and his wife moved to Babylon, N. Y. , where they lived on a baronial-sized estate. Though Keith maintained his extensive interests in Costa Rica, he sold his banana properties there to United Fruit. During the last 2 decades of his life he focused his interest on the Central American International Railroad project. This would have meant a rail link from Mexico to Panama. Keith felt this would spur political integration, population movement, and economic growth. Though never completed, by the time of his death on June 14, 1929, the railroad had 887 miles of track laid in Guatemala and El Salvador, and the former country had an inter-oceanic rail link. The growth fostered by the later Pan-American Highway vindicated the accuracy of Keith's then visionary ideas.
Though only 5 feet 5 inches tall and 140 pounds, Keith made up in energy and courage what he lacked in size.
In 1883 Minor Keith married Cristina Castro Fernández, daughter of Dr. José María Castro Madriz, niece of President Fernández, and cousin-in-law of Soto. His nephew-in-law Rafael Iglesias Castro served twice as President of Costa Rica (1894-1898, 1898-1902).