Harington, who played fan favourite Jon Snow on the epic TV fantasy series, had to take a hiatus from work to "really concentrate" on himself after the show ended in May 2019.
The actor told SiriusXM's "The Jess Cagle Show", the "nature" of the violent, intense drama "directly" affected his mental state.
"I went through some mental health difficulties after (Game of) Thrones, and during the end of Thrones, to be honest," Kit shared. "I think it was directly due to the nature of the show and what I had been doing for years."
The actor, who wed "Game of Thrones" co-star Rose Leslie in 2018 and welcomed their first child in early 2021, said he's "really happy" he took off an entire year to focus on his own well-being.
However, his planned comeback did not go quite as expected.
"Just when I was wanting to come back to work the pandemic hit," he explained.
He was, however, able to film an episode of upcoming Amazon series "Modern Love", which is based on The New York Times' column of the same name.
"You don't have to live in that intense place all the time," Harington said of the welcome change of pace. "Why don't you do something that takes the weight off? Why don't you do something fun?"
In the episode, Kit plays a man who meets the girl of his dreams - played by Lucy Boynton - but their plan to reconnect two weeks later is derailed by the COVID lockdown.
"Modern Love" season two debuts on 13 August (21) on the U.S. streaming site.