Toobworld Central asserts that Agent 99's real name is Mandy Stephenson (from an episode of 'The Man From U.N.C.L.E.') and that she used the alias of Lauren Holden while she was a United States senator and having a once-a-year fling with a former Red Sox player (as seen in an episode of 'Cheers').
Just an O'Bservation - It was nice that 99 was empowered with the role of senator when the series was revived, but it didn't make any sense. Nobody gets elected to such a major office without being thoroughly vetted by the press and by their opponents. Everything in her past would have come out - her career as a spy, her affair with Sam Malone.
What they should have done was made her a National Security Advisor instead.