He had been an astronaut on Mars Probe 6, partnered with Jim Daniels. They landed on Mars and encountered aliens who — not realising that their touch was fatal to humans — accidentally killed Daniels.
Afterwards, during the launch of Mars Probe 7, he became head of the new Space Security Department. Carrington plotted with Sir James Quinlan to capture three of the aliens. While Quinlan wanted the political coup of being involved in "first contact" with alien ambassadors, Carrington was using him for his own agenda: he believed the aliens were a threat to Earth and wanted to convince the world to go to war against them, believing this to be his "moral duty". Using loyal members of his regiment, a criminal gang headed by Reegan, and the scientists Bruno Taltalian and Lennox, he was able to force the aliens into attacking government installations (and to kill Quinlan to cover his tracks).
Carrington become more obviously unhinged as the conspiracy went on, with Ralph Cornish noticing he was insane after the United Nations Security Council decided not to launch a first-strike on the aliens. He eventually arrested UNIT because he thought they were working with the aliens and seized control of the Space Control Centre, planning to expose the aliens on live television to terrify the general public. He was stopped after the Doctor, Brigadier Lethbridge-Stewart and the newly-released UNIT troops arrived to rescue the aliens. The fight went out of him when he realised he had lost, and he was arrested.