Your code never returns to the event loop so the QNetworkReply objects will not be deleted until the entire queue is finished

I really do not know why you are trying to build a blocking system rather than simply connect the QNetworkAccessManager::finished() signal to a slot to handle the finished reply. The "problem" would then be a non-issue. I also see no reason for the mutex in the RequestFinishedHandler: yes, QNetworkAccessManager uses threads internally, but your code remains single-threaded.