The 55-year-old actor, who joined Tom Cruise on set in the iconic action movie franchise in 2006, admitted despite dreaming of a role in a Hollywood blockbuster, he was deeply unhappy at the time.
"What I was ultimately dealing with was depression, which I was trying to manage by anaesthetising myself," he told Variety.
"It was less of being alcoholic, more being dependent on the sensation."
The Hot Fuzz and Shaun of the Dead star recalled being "hung over" for his first day on set with Tom.
"(I was) slightly hungover and feeling a little bit wired," he said.
"I remember not being particularly present, because I was so inside my own head that it was hard to actually relax and enjoy it."
Once he got back from the shoot, he headed straight for the hotel bar in what he described as a "nihilistic, self-destructive impulse".
His second outing in the franchise, for 2011's Ghost Protocol, helped him start to overcome his personal struggles.
Pegg had a greater role for that film, while the previous movie's director JJ Abrams and his successor Brad Bird - and the production team - were aware of his battles off-screen.
"They had a sober companion for me - and just really looked after me," he recalled.
"I felt very taken care of and felt valued, because they bothered to do that."
It was a turning point for Simon, and gave him something to focus on when he needed it the most.
"I kind of found this joy in looking after myself and just realizing that if I ate well and went to the gym I could actually feel good," he added.
"So I credit Mission with rescuing me - because it gave me real focus at a time when I needed focus. And I just had the best time on Ghost Protocol."
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