Ambassadors work for the U.S. State Department. They represent the interests of the president through the secretary of state. Many ambassadors are FSOs who have worked up through the ranks of the Foreign Service.
Those who are appointed as ambassadors have already succeeded in their individual careers. While many ambassadors have worked in the Foreign Service in some capacity, many others have established themselves in other ways. They have worked as directors in other government agencies and as members of the U.S. Congress. They've served on the faculty of colleges and universities. They have directed philanthropic organizations and run large companies. Before getting an ambassadorship, ambassadors may have already had a great deal of experience with a particular country, possibly having served as Deputy Chief of Mission (the second in command of an embassy). Or they may have been involved in negotiating international agreements or in establishing new markets for the country.
After being nominated for an ambassadorship by the president, nominees are confirmed by the Senate. Positions with the Foreign Service are rotational, so the length of an ambassador's term varies. Most ambassadors only serve a few years with an embassy. After leaving a post, they may go on to serve as ambassador at a U.S. embassy in another country. Some career ambassadors spend several years moving from embassy to embassy. Many ambassadors have published books on foreign policy, international affairs, and world trade.
Read publications such as The Foreign Service Journal (http://www.afsa.org/foreign-service-journal) and STATE magazine (https://www.state.gov/resources-bureau-of-human-resources/#statemag) to learn more about the field.
Visit https://careers.state.gov/officer to learn about careers in the Foreign Service.
Conduct information interviews with ambassadors and Foreign Service officers and ask them for advice on preparing for and entering the field. The U.S. Department of State offers the Diplomats in Residence Program, in which career FSOs located throughout the United States provide advice to young people about careers. Visit https://careers.state.gov/connect/dir/ for more information.