After school, he clerked for John Minor Wisdom in New Orleans. In 1972, the couple moved to Washington, D.C. where he clerked for Supreme Court justiceByron White and his wife became a news-desk editor at the CBS News bureau. After his one-year clerkship, he accepted a position as aide to Minnesota SenatorWalter Mondale who assigned him the task of soliciting support for legislation to curb filibusters. He became close friends with fellow Mondale aide Michael Berman. In 1975, he was hired by Joseph A. Califano Jr. and joined the law firm of Williams, Connolly & Califano. Soon after, Mondale was chosen as Jimmy Carter's presidential running mate and Barnett took a leave of absence from his law firm and went to Atlanta to help run Carter's campaign. After Carter's win, Barnett returned to his law practice. He made a name for himself defending white-collar clients including Toyota distributor, Jim Moran and former head of Fannie Mae, Franklin Raines. In 1984, Barnett helped to prep Mondale's running mate, Geraldine Ferraro, for her debate against George H.W. Bush; and defended her from accusations about her husband's alleged Mob affiliations and questionable tax returns. He also represented David Stockman and Kitty Dukakis, helping both to secure lucrative contracts with book publishers. In 1992, he again served as a debate coach this time for Bill Clinton in his successful presidential campaign. Returning to private practice, he found many customers eager to have him represent them in securing lucrative contracts and advances for their books including Secretary of stateJames Baker; former Vice PresidentDan Quayle and that of his wife Marilyn Quayle; James Carville and his wife Mary Matalin. He also expanded into representing television news people in contract negotiations including his wife, Rita Braver, Susan Mercandetti, Robin Lloyd, Chris Wallace, Andrea Mitchell, Brit Hume, Wolf Blitzer, and Ann Curry, In 1992, he became the personal attorney of Hillary Clinton and assisted the Clintons when their aide Vince Foster committed suicide. After criticism arose as to whether it was appropriate for Barnett to represent so many newscasters and journalists who were reporting on the White House while also serving as lawyer to the Clintons; Barnett resigned from representing the Clintons turning his work over to fellow Williams, Connolly & Califano attorney David E. Kendall. He represented George Stephanopoulos in an incriminating book about the Clintons. In 1997, after his wife resigned from CBS, he went back to representing the Clintons, and helped to negotiate the sale of their post-presidential memoirs; and helped former Clinton officials to secure new employment ; and in 2004, Barack Obama with the reissuance of Dreams From My Father and sell The Audacity of Hope and Change We Can Believe In. Despite this, he remained a Hillary Clinton supporter and assisted her preparing for all her debates against Obama. Barnett prided himself on representing clients regardless of political affiliation and also assisted outgoing Republican officials, Karen Hughes and Ari Fleischer, in finding new positions. He also served as a practice debate opponent for many Democratic Presidential and Vice-Presidential candidates.
