Livianus was a well connected and influential figure in Late Republican politics. A member of the aristocratic party, brother of the tribune Marcus Livius Drusus and son of the consul Marcus Livius Drusus, he was adopted into the Aemilii Lepidi. His influence was such that he was able to intercede with Lucius Cornelius Sulla on the young Julius Caesar‘s behalf, getting Sulla to spare Caesar’s life. He was also married to Cornelia, Sulla’s daughter.
Around 91 BC Livianus succeeded his brother Marcus Livius Drusus as one of the pontifices in the College of Pontiffs. He served with distinction in the Social War (91–87 BC), probably serving as legate under Quintus Caecilius Metellus Pius. He achieved some victories, most notably he was credited with killing the general of the Marsi, Quintus Poppaedius Silo, during the storming of Venusia.
Although having failed once to be elected praetor, Livianus tried again, achieving the office by 81. He ran for the office of consul in 77, achieving it only after Gaius Scribonius Curio withdrew his candidature for that year in favor of Livianus. Neither Mamercus nor his consular colleague accepted a proconsular command in Hispania to help Quintus Caecilius Metellus Pius in the Sertorian War. There is no evidence that he obtained a provincial command after his term was concluded.
Following his term as consul Livianus was probably a promagistrate serving under Marcus Antonius Creticus in 74, who had been given an extraordinary commission to clear the Mediterranean Sea of pirates operating from Crete. By 70, he may have been Princeps Senatus, although the evidence is inconclusive. He was called as a hostile witness against Gaius Cornelius in 65, as part of the events surrounding the First Catilinarian Conspiracy.