A career prosecutor, Letten served as U.S. Attorney for the Eastern District of Louisiana for more than eleven years.

[2] In that position Letten was best known as the lead prosecutor in the racketeering trial of former Louisiana Governor Edwin Edwards.

"[4] The Times-Picayune listed the following as the notable public corruption cases in which the U.S. Attorney's Office achieved convictions under Letten's leadership: "ex-Jefferson Parish Judges Ronald Bodenheimer and Alan Green; former Orleans Parish School Board President Ellenese Brooks-Simms; ex-state Sen. Derrick Shepherd; former St. Tammany Parish Councilman Joe Impastato; a whole bunch of folks who worked for or did business with the Orleans Parish School Board; several high-profile associates of ex-Mayor Marc Morial; and ex-New Orleans City Councilman Oliver Thomas, who had been pegged as a front-runner in the 2010 mayor's race.

[8] In September 2009, the Times-Picayune praised Letten and the FBI for "bringing to justice" Bill Hubbard, who resigned as Saint John Parish president after receiving bribes of $20,000 from contractors.

[4] In 2010, the conservative provocateur videographer/prankster James O'Keefe, "who specialized in often deceptively edited undercover camera work to expose what he considers to liberal hypocrisies," pleaded guilty to federal misdemeanor charges of entering the New Orleans offices of Senator Landrieu disguised as a telephone repairman and attempting to tamper with the office's phone system under false pretenses.

"[4] Senator Mary Landrieu and Attorney General Eric Holder paid tribute to Letten's service.

[4] In February 2013, Letten was appointed assistant dean of experiential learning at Tulane University Law School, his alma mater.

[3][13] In November 2015, Letten joined the New Orleans office of the law firm Butler Snow LLP as of counsel.