A long-awaited watchdog report provided a sweeping and detailed rebuke of actions taken by former FBI Director James Comey to publicize details about the politically charged investigation into Hillary Clinton’s use of a private email server, but found no evidence that the probe was affected by bias or other political considerations.

The 500-page report also faulted Obama-era Justice Department leaders for not taking a firmer hand with Mr. Comey, and slammed multiple FBI employees involved in the investigation for sending...