He served as a Democratic member of the Louisiana State Senate, representing a portion of Orleans Parish for four terms.

[1] Failed bid for mayor of New Orleans in 1960, despite winning nearly all of the black vote,[2] but losing the election to Victor H. Schiro[1] On April 24, 1978, Duplantier was nominated by President Jimmy Carter to a seat on the United States District Court for the Eastern District of Louisiana vacated by Judge Roger Blake West.

He assumed senior status on March 6, 1994, and served until his death, in New Orleans.

[1] Duplantier and two other Louisiana Democrats, former State Treasurer Mary Evelyn Parker and former State Representative Risley C. Triche of Napoleonville in Assumption Parish, were interviewed for the 2001 book Welfare Racism: Playing the Race Card Against America's Poor.

The three testified to their personal knowledge of racism in 1960–1961 in Louisiana against African American public assistance recipients.