The Juggernauts were created by installing the DNA of humans into the mechanical shells of Mechanoids, robots whose original purpose had been to serve humans and prepare planets for human colonisation. These cybernetic creations were to be self-serving, self-repairing and self-replicating. The Juggernauts were capable of creating more of themselves as long as the right resources were available. These Juggernauts were divided into different types which included ones that focused exclusively on the manufacturing process while another security class dealt with threats or intruders.
The Daleks recruited the Sixth Doctor in order to get his help in capturing Davros. When he called in the Dalek forces, they swarmed in and killed the Daleks loyal to Davros as well as the few Juggernauts present. It was at this point that more Juggernauts arrived to aid their master and succeeded in repelling the Dalek forces. However, the Doctor's companion Mel used a backdoor command to take control of the Juggernauts and told them to designate Davros as a threat. After they turned on him, the newly arrived Dalek reinforcements succeeded in wiping out the Juggernaut threat. (AUDIO: The Juggernauts)