The appointment was announced the day after the United Nations said more than 1,000 people were killed in Iraq in July making it the country's bloodiest month since 2008.
Mladenov "will be turning his attention to that" as other envoys to Iraq have had to, said UN spokesman Martin Nesirky.
The new envoy takes over from German diplomat Martin Kobler leading the UN Assistance Mission for Iraq whose main task now is to help the government with efforts to reconcile with opposition parties and with elections.