Republican George LeMieux officially began his 16-month term in the Senate on Thursday when he was sworn in to take the seat once held by Sen. Mel Martinez (R-Fla.).

LeMieux, who ran Florida Gov. Charlie Crist’s (R) 2006 campaign, was tapped by his former boss Aug. 28 to serve out Martinez’s term. LeMieux has vowed not to run for a full term next year, but the man who appointed him — Crist — will run for the seat in 2010.

At 40 years old, LeMieux is now the Senate’s youngest Member and the fifth to be appointed in this session, joining Democratic Sens. Roland Burris (Ill.), Michael Bennet (Colo.), Kirsten Gillibrand (N.Y.) and Ted Kaufman (Del.).

The former Broward County Republican Party chairman graduated from Georgetown University’s law school in 1994 and recently served as chairman of the board of directors at the law firm of Gunster, Yoakley & Stewart.

Martinez, who delivered his farewell address Wednesday, announced last month that he would vacate his seat after less than one term. He had already decided against a re-election bid in 2010.

Martinez enjoyed plum spots on the Armed Services; Banking, Housing and Urban Affairs; Commerce, Science and Transportation; and Aging committees. It’s unclear whether LeMieux will be able to maintain the same portfolio.