Colonel Luther Manning had seen the battle in Vietnam, El Salvador, Iran and Persian Gulf. Luther was ask to joined the Cybertek program by Cybertek Systems Inc. but refused. In an alternate future timeline, he was injured in battle and became Deathlok. He was transported back in time and joined forces with Captain America.
Luther Manning is the first Deathlock ever. He was however from a different reality, so does not count as the first Deathlock on Earth 616 (or main Marvel Universe). He first appeared in Astonishing Tales #25.
Mayor Story Arcs
In the past, Luther eventually started having dreams of becoming Deathlok. He was eventually confronted by the being know as Timestream who infused him with temporal energy which he later triggered turning him into Deathlok earlier in the timeline then he would've become.
Going to the Main Marvel Universe
As a cyborg Luther was incapable of resisting Timestream's order after he reprogrammed him using a CD of the original memory of his alternative reality counterpart. Timestream then used him to go to the past to kill Captain America but he was opposed by Godwulf (a resident of Earth-7484), the current Deathlok of Earth-616 (Michael Collins) and Siege. It was than revealed that Timestream was going to turn Earth-616 into a replica of his world but failed do to the interaction of these heroes.
However Luther was still under his control and Timestream's next plan was to use him to kill the current Deathlok, while both of them fought, Luther came out as the victor. Since Collins was a pacifist at heart and could not fight back Luthers warrior skills. Just when Luther was about to kill him he used his link as both cyborg to enter Luther's mind and make him see that Timestream was using him. Luther now saw this horrible nightmare and blasted himself in the head killing himself. Upon his death the energies that Timestream used to turn him into Deathlok vanished, reverting his body back to human. Justice Peace a member of the Time Variance Authority then showed up and claimed that he would get a proper burial for Luther's body in the present.
Some time later Deathlok was recruited by X-51 (Machine Man) of Earth-9997 into his group of Heralds. At the end of their mission, this X-51 used a device to create a new body for Manning from his DNA and offered him a chance to regain his human life.
Recently, Deathlok was kept under S.H.I.E.L.D protection until the villain the Owl kidnapped it, and put it up for auction as a weapon. The criminal known as The Hood and his now organized group of villains bought him, and robbed a bank. Deathlok was seemingly destroyed during this event.
The first replica of the original Deathlok, called an Exact Medical Replica, was created while he was working for the CIA of Earth-7484. The EMR was used to observe Deathlok's systems while he was on assignment. The unit no long functioned when Deathlok was transported to Earth-616 by Godwulf. When Luther Manning's human clone saw the unit, he was repulsed by the sight of it and ask the unit be destroyed.
The second replica of Deathlok was a robot that attacked the Thing. Thing destroyed this Deathlok while believing it to be the original, though it was later realized that it was only a robot replica created by the Brand Corporation.