J. Reuben Clark is the first Mormon appointed to federal government service (U.S. State Department's assistant solicitor). He serves as the department's solicitor from 1910 to 1911. No Mormon serves in an equivalent legal position until Rex E. Lee's appointment as U.S. Solicitor General (1981-85).
[The Mormon Hierarchy - Extensions of Power by D. Michael Quinn, [New Mormon History database (http://bit.ly/NMHdatabase)]]