The present invention relates to use of carbon-based sorbents such as activated carbon for controlling vapor phase mercury emissions in coal-fired power plant flue gases or waste incineration flue gases. The invention features new methods that improve the performance of carbonaceous sorbents for mercury control compared to the previous methods. One method consists of injecting the sorbent (consisting of at least some carbonaceous material) into the flue gas where the flue gas temperature is at a value above about 1100° F. and below about 2000° F. The invention also consists of injecting the sorbent within or upstream of an economizer section in a coal-fired power boiler. Injecting the sorbent at the locations indicated above, rather than further downstream, provides increased residence time for contact between the flue gas and the sorbent resulting in increased mercury capture, improved sorbent utilization, and improved fly ash properties.