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Crist made the formal announcement in Tallahasse this morning.
Calling him a "great man," Crist said Lemieux "will serve well, the people, again, in public service."
LeMieux, 40, will take over when Martinez, who announced earlier this month that he would be leaving his seat a year early, officially resigns.
"George is bright, capable, and an accomplished administrator," Martinez said in a statement congratulating LeMieux.
Born and raised in Broward County, LeMieux returned to Fort Lauderdale to practice law after graduating from Georgetown University School of Law. He became Chairman of the Broward County Republican Party in 2000.
When Crist was Florida's Attorney General in 2003, he brought LeMieux in as the state's Deputy Attorney General.
LeMieux served as Crist's campaign manager in 2006, and was largely credited with Crist's win. Crist referred to LeMieux as his "Maestro" after the victory.
LeMieux served as Crist's Chief of Staff until 2007, when he left to practice law again. He's currently the chairman of the law firm of Gunster Yoakley & Stewart, P.A.
The pick ends weeks of speculation over who Crist would pick to fill the seat, which he hopes to occupy after the 2010 election. Crist had narrowed his list down to nine candidates to fill the seat, including former U.S. Rep. Clay Shaw, State Rep. Jennifer Carroll, former Sen. Dan Webster, and former U.S. Attorney Roberto Martinez.
Crist had been scheduled to come to South Florida today for the installation of a judge.