Mogherini, 41, began the day as front-runner but Poland and Baltic states voiced misgivings about her inexperience and her emollient attitude towards Russia since its annexation of Crimea in March.
But the Italian premier said none of the other EU member states had vetoed the decision of Mongherini's candidature.
"We have been here for 57 years and we have never vetoed anyone, and we never had to withstand it (veto), not even today. If this was your question, then this is my answer," Matteo Renzi told reporters at the end of the summit.
Meanwhile, the leaders did agree to step up sanctions against Russia over separatist violence in eastern Ukraine, shortly after Washington also tightened the screws.
The EU will expand sanctions against Russia to target companies that undermine Ukrainian sovereignty and will ask the EU's European Investment Bank and the European Bank for Reconstruction and Development to suspend new lending.
Shrugging off criticisms of Italy's reluctancy to impose EU sanctions on Moscow, Renzi told media the country was on the same "position" as other European member states.
"Italy is on exactly the same position as other EU states," Renzi said.
"The point is that, if anyone has a doubt about Italy's political stance, let it be known that it is indeed the same as the other member states. Therefore those who call into question Italy, are also calling into question France, Germany, Great Britain, the G7. All the documents that we have drafted, we have drafted them together," he added.