Mannen and his brothers, John Gibson “Gip,” James, and Joseph, were brought up on a cattle ranch south of Smiley, Texas, in Gonzales County, Texas. In 1871, John Wesley Hardin, a cousin of the Clements, visited their ranch and participated in a cattle drive to Kansas with Mannen and James Clements. Mannen killed brothers Adolph and Joseph Shadden during the drive, who had disputed his authority just as the herd crossed the Red River into Indian Territory. He was later jailed in Kansas by Bill Hickok but was released at the request of John Wesley Hardin, who had become friends with Hickok.
In October 1872, Mannen helped Hardin escape from a jail in Gonzales County, Texas, by slipping him a file, then pulling him between the jagged bars by a lariat. In the years that followed, Mannen, along with brothers Joe, Jim, and Gip, participated in the Taylor-Sutton Feud with their cousin, Hardin. In 1877 Clements found himself in jail in Austin, Texas, along with Hardin, Bill Taylor, Johnny Ringo, and members of the Sam Bass gang. After John Wesley Hardin was sent to prison, Mannen was one of the few people that ever visited him while he was there. He also helped Hardin’s wife, Jane, and their children.
By 1880 Clements was suspected of cattle rustling, and he had accumulated vast horse and cattle herds on his McCulloch County Ranch. About two years later, Clements hired none other than Killin’ Jim Miller to work on his ranch. While there, Miller became good friends with Emmanuel’s son, Emmanuel “Mannie” Clements, Jr., and Mannen’s daughter, Sallie. Miller and Mannie Clements would later be embroiled in the Frazer-Miller Feud in Pecos, Texas.
In spite of his past, Mannen Clements ran for sheriff of the newly formed Runnels County in early 1877 in a hotly contested campaign. Despite his efforts, he lost. On March 29, 1887, Mannen was shot and killed in the Senate Saloon by Ballinger City Marshal Joseph Townsend. Not long afterward, Townsend was riding home one night when he was swept from the saddle by a shotgun fired out of the dark. The ambusher was never identified, but Jim Miller was widely suspected. Though Townsend survived, he lost an arm.