State Rep. Jennifer Carroll, who was considered for the position, said Crist told her he is choosing LeMieux. LeMieux is Crist's closest political adviser and the governor's pick shows he wants someone who thinks like him to hold the Senate seat Crist hopes to win in the November 2010 election.
As LeMieux said after interviewing for the position: "I'm a Charlie Crist Republican."
Martinez congratulated LeMieux on his appointment.
"George is bright, capable, and an accomplished administrator," he said in a written statement. "My staff and I stand ready to ensure a smooth transition."
Florida GOP leaders applauded the appointment.
"Once again, Charlie Crist has demonstrated his commitment to serving Floridians, by appointing George LeMieux who is well qualified, a dedicated public servant, conservative Republican and an excellent choice!" exclaimed Jim Greer, chairman of the Republican Party of Florida.
But the Democratic Senatorial Campaign Committee immediately assailed the appointment.
"Charlie Crist came as close as he could to appointing himself to this position," the DSCC said in a written statement. "At a time when so many Floridians are unemployed, and many others facing foreclosures, we have learned nothing is beyond the pale for Charlie Crist."
"Today marked another Charlie Crist choice that significantly impacts the state of Florida but is ultimately about promoting himself," the group added.
LeMieux served as deputy attorney general under Crist and left that job to run Crist's 2006 campaign for governor. Once elected, Crist picked LeMieux to serve as chief of staff. LeMieux left the position in December 2007 and has since worked for the Tallahassee law firm Gunster, Yoakley & Stewart.
Crist has continued to seek LeMieux' advice on political and policy issues.
Crist had a final list of nine candidates for the appointment. It included former U.S. Rep. Clay Shaw, Mike Bilirakis and Lou Frey. Crist also passed on a chance to please party conservatives by picking former Sen. Dan Webster. The governor also could have tried to win points with certain constituents by picking Carroll, a black woman and retired Navy officer, or former U.S. Attorney Roberto Martinez, who was born in Cuba.
Other candidates considered were University of North Florida president and former Jacksonville Mayor John Delaney and Jim Smith, a Tallahassee lobbyist who is a former attorney general and secretary of state.
Martinez announced in December he wouldn't seek a second term, and earlier this month said he would step down once Crist found a replacement. Martinez was the first Cuban native to be elected to the Senate, and previously served as President George W. Bush's housing secretary.
LeMieux was born and raised in Broward County and was twice elected the county's Republican Party chairman. In 1998 he unsuccessfully ran for a state House seat.
Crist faces former House Speaker Marco Rubio in the Republican primary next August. U.S. Rep. Kendrick Meek is the only major Democrat in the race, though Rep. Corrine Brown has formed a committee to explore a Senate run.
The Associated Press contributed to this report.