Stephen Willeford, who lives near the Texas church targeted in the bloody attack which left 26 people dead, grabbed his own rifle and rushed to confront Kelley after hearing about the shooting.
The 55-year-old plumber came face-to-face with the crazed ex-solder outside the church and shot him through a gap in his body armour.
Mr Willeford and another passing resident, Johnnie Langedorff, then went after the gunman in a 100mph car chase after Kelley grabbed a hostage tried to make his getaway.
Freeman Martin, regional director of the Texas Department of Public Safety, said: “The suspect dropped his rifle, which was a Ruger AR assault-type rifle, and fled from the church. The man then chased the gunman.”
Mr Langedorff said: “He got a little bit of a jump on us. So we were doing about 95 miles per hour, going around traffic and everything.
“He just lost control and that’s when I put the vehicle in park. The other gentleman jumped out and had his rifle drawn on him and he didn’t move after that.”
Mr Langendorff told KSAT television he was acting on instinct to try to prevent the suspect from escaping.
He said Mr Willeford asked for help and got into Mr Langendorff’s truck. They took off at high speed in pursuit of the gunman on a nearby highway as police were heading in the opposite direction toward the church.
Mr Langendorff said the gunman eventually lost control of his vehicle and crashed.
He said Mr Willeford walked up to the vehicle with his gun drawn and the suspect did not move.
Mr Willeford then stood guard over the gunman until police arrived five to seven minutes later.
Mr Langendorff said: “I was strictly just acting on what’s the right thing to do.”
Kelley’s body was found close to his crashed vehicle. He died from a bullet wound which police belief was self-inflicted.
The hostage is understood to have escaped unharmed.
US President Donald Trump praised Mr Willeford for preventing further casualties but brushed aside questions about whether the US needs to consider tightening gun laws.
Mr Trump said: “Fortunately somebody else had a gun that was shooting in the opposite direction, otherwise it would have been – as bad as it was – it would have been much worse.”