He was appointed British vice-consul in 1842. While serving as vice-consul, he advocated the takeover of California by the British government; the proposal was flatly rejected. Forbes was informed that Great Britain had no desire to interfere in California politics, but that she would look with great disfavor upon such interference by any other power. This sentiment was communicated to Thomas O. Larkin, United States consul, but did little to dissuade the U.S. from annexing California in 1846.
