Born in Inverness, Scotland to John Alonzo Forbes and Marta Rodriguez, he emigrated to Argentina around the age of 12 with an uncle who owned a shipping line.
[8] While serving as vice-consul, he advocated the takeover of California by the British government; the proposal was flatly rejected.
This sentiment was communicated to Thomas O. Larkin, United States consul, but did little to dissuade the U.S. from annexing California in 1846.
[9] The Forbes family lived on Rancho Potrero de Santa Clara, a 1,939-acre (7.8 km2) Mexican land grant near San Jose received from Governor Manuel Micheltorena in 1844.
[12] He was also an early investor in the New Almaden quicksilver mine with Alexander Forbes, British consul to Mexico and author of one of the first histories of California in the English language.
However, José de los Reyes Berreyesa also laid claim to the mine on the basis that it lay on Rancho San Vicente.
A complex legal case involving Mexican laws and the Treaty of Guadalupe Hidalgo described by The New York Times as "one of the most remarkable civil trials in this or any other country",[13] United States v. Andres Castillero eventually reached the United States Supreme Court whereupon it was finally decided in 1862 that, the mine was on the Larias grant, and that the furnaces and improvements of the company below the hill were on the Berreyessa grant.
This, coupled with the delays in its opening which allowed competitors into the market thus dropping flour prices from a lucrative $50/barrel to a mere $5,[16] meant that it was not generating enough income and eventually Forbes was forced into bankruptcy, much of his money also being tied up in litigation with regards to the mine.
[17] Los Gatos historian William A. Wulf describes Forbes as "a suede-shoe man [i.e., a devious individual].