Mark Elgin was the public relations director for Global Chemicals, and also served Jocelyn Stevens in the capacity of an assistant sometimes. He was also initially one of the employees not directly controlled by BOSS. Not being controlled by BOSS meant that he was able to notice that Stevens and others were behaving strangely.
When Ralph Fell told Brigadier Lethbridge-Stewart that Global Chemicals had no equipment with which to rescue Jo Grant and Bert Pritchard from the Llanfairfach mine, Elgin knew he was lying. Jo was rescued nonetheless, although Bert was killed.
Suspecting something was amiss, he followed Fell later and caught him attempting to kill the Doctor and Jo by flooding the pipe they were in with toxic waste while they were returning from exploring the mine. He was able to overpower Fell and stop him, rescuing the pair. Fell escaped, and then later, seemingly inexplicably, ran past the three and leaped to his death.
After the Doctor and Jo left and returned to the Nut Hatch, an enraged Elgin finally confronted Stevens, who locked him inside his office and allowed BOSS to brainwash him. Shortly after this, he came down with appendicitis and had to be rushed to the emergency room. He was freed from BOSS' control when the computer was destroyed by Stevens. Years later, Elgin participated in the documentary Global Conspiracy and helped bring to light the crimes of Global Chemicals.
- Actor Tony Adams did actually come down with appendicitis during the filming of The Green Death and was consequently unable to complete filming. Originally, the brainwashed Elgin was to be sent to kill the Doctor, be freed from his mind control, and inform the Doctor and the Brigadier of BOSS' plans. However, when Adams had to be rushed to the emergency room, a hasty rewrite divided Elgin's role in the latter portion of the serial between Mike Yates and James.
- Adams' illness was used in Global Conspiracy to canonically explain Elgin's sudden absence from the story, when Adams reprised his role for the fictional documentary.
- The novelization of The Green Death follows the original script, and has Elgin being killed by BOSS in place of James, who doesn't appear.