Cooley's life changed on May 13, 1875 when his friend, Tim Williamson was arrested by Deputy Sheriff John Worley on the suspicion of cattle rustling. While Worley escorted Williamson to jail, an angry mob of German cattlemen abducted the prisoner and shot him to death. This incident marked the beginning of the Mason County War in Texas, that pitted the German cattlemen against the native-born Texans. 

Scott Cooley blamed Worley for Williamson's death, believing him to have been in collusion with the German ambushers. However, he  waited for the ambushers to be arrested, but when no indictments were made against them, he took matters into his own hands.

He soon recruited several men to help him, including John and Mose Beard, George Gladden, and Johnny Ringo and began to avenge the death of Tim Williamson.

Cooley and his "posse" first went to Worley's home where he found the deputy working on his well with an assistant, who had been lowered over the side. Cooley shot Worley dead, and the well worker, clinging to the rope, tumbled to the bottom of the well. Cooley then cut scalped Worley, proudly displaying his prize to the Germans.

They then killed Peter Bader, who was believed to have been Williamson's killer, before tracking down murdering another man named Daniel Hoerster, whom they suspected of having been part of the ambush group. The Germans retaliated when a posse led by Sheriff John Clark, ambushed Moses Baird and George Gladden, killing Baird and seriously wounding Gladden. The posse included Charles Bader, brother to Cooley's second victim, Peter Bader.

Cooley and his allies then began to kill a number of the ambushers, which was retaliated by the hanging of  two of Cooley's confederates. The killing continued in both directions for the next several months and the Texas Rangers did little, as many of them were friends of Cooley's.