In January 2009, Coates won a unanimous U.S. Supreme Court decision securing broader immunity for prosecutors in criminal trials (Van de Kamp v. Goldstein). Both state and federal prosecutors have long enjoyed immunity for actions connected with criminal charges they bring and cases they try—in short, for actions associated with the judicial phase of the criminal process. Coates got that rule expanded to cover prosecutors in their supervisory or administrative roles, an integral part of running a prosecutor's office. In this case it shielded John Van de Kamp from liability for decisions he made while he was Los Angeles County District Attorney; he had faced allegations of failing to implement an effective system to track the veracity of jailhouse informants.

Public trust in the prosecutor's office would suffer if officials had to constantly look over their shoulders to guard against personal liability, the high court ruled. Defending managerial decisions, often years after they are made, could impose "unique and intolerable burdens upon a prosecutor responsible annually for hundreds of indictments and trials," Justice Stephen Breyer wrote in citing an earlier holding.