22 USC 3941 - General provisions

(a) Citizenship requirement 
Only citizens of the United States may be appointed to the Service, other than for service abroad as a consular agent or as a foreign national employee.
(b) Examinations 
The Secretary shall prescribe, as appropriate, written, oral, physical, foreign language, and other examinations for appointment to the Service (other than as a chief of mission or ambassador at large).
(c) Veteran or disabled veteran 
The fact that an applicant for appointment as a Foreign Service officer candidate is a veteran or disabled veteran shall be considered an affirmative factor in making such appointments. As used in this subsection, the term veteran or disabled veteran means an individual who is a preference eligible under subparagraph (A), (B), or (C) of section 2108 (3) of title 5.
(d) Career and noncareer appointments 

(1) Members of the Service serving under career appointments are career members of the Service. Members of the Service serving under limited appointments are either career candidates or noncareer members of the Service.
(2) Chiefs of mission, ambassadors at large, and ministers serve at the pleasure of the President.
(3) An appointment as a Foreign Service officer is a career appointment. Foreign Service employees serving as career candidates or career members of the Service shall not represent to the income tax authorities of the District of Columbia or any other State or locality that they are exempt from income taxation on the basis of holding a Presidential appointment subject to Senate confirmation or that they are exempt on the basis of serving in an appointment whose tenure is at the pleasure of the President.