He worked for Time magazine as a correspondent, bureau chief, and senior editor. He was an international banker with both Ameritrust in Cleveland, Ohio and First Interstate Bank in Los Angeles and traveled extensively overseas. Prior to his career as a journalist and historian, Gwynne was a French teacher at Gilman School in Baltimore, Maryland.

He lives in Austin, Texas with his wife, the artist Katie Maratta. He also has a master's degree in writing from Johns Hopkins University, where he was awarded a graduate fellowship and studied under novelist John Barth.

He majored in history at Princeton University and graduated in 1974. He attended secondary school at The Hill School. Gwynne was born in Worcester, Massachusetts and grew up mainly in New Canaan, Connecticut. He holds a bachelor's degree in history from Princeton University and a master's degree in writing from Johns Hopkins University. For Mackenzie on the southern plains, Comanches were the obvious target: No tribe in the history of the Spanish, French, Mexican, Texan, and American occupations of this land had ever caused so much havoc and death. They were Comanches, Kiowas, Arapahoes, Cheyennes, and Western Sioux.

The hostiles were all residents of the Great Plains all were mounted, well armed, and driven now by a mixture of vengeance and political desperation. Victorious in war, unchallenged by foreign foes in North America for the first time in its history, the Union now found itself unable to deal with the handful of remaining Indian tribes that had not been destroyed, assimilated, or forced to retreat meekly onto reservations where they quickly learned the meaning of abject subjugation and starvation. He had been dispatched to kill Comanches in their Great Plains fastness because, six years after the end of the Civil War, the western frontier was an open and bleeding wound, a smoking ruin littered with corpses and charred chimneys, a place where anarchy and torture killings had replaced the rule of law, where Indians and especially Comanches raided at will. For now, Mackenzie was the instrument of retribution. He was new to this sort of Indian fighting, and would make many mistakes in the coming weeks. Neither did he have the faintest idea that he would be the one largely responsible for defeating the last of the hostile Indians. As he moved his men across the broken, stream-crossed country, past immense herds of buffalo and prairie-dog towns that stretched to the horizon, Colonel Mackenzie did not have a clear idea of what he was doing, where precisely he was going, or how to fight Plains Indians in their homelands. But it was Mackenzie, not Custer, who would teach the rest of the army how to fight Indians. “In roughly that same time period, while General George Armstrong Custer achieved world fame in failure and catastrophe, Mackenzie would become obscure in victory.

Gwynne empire of the summer moon