The actor, 47, was catapulted into the limelight in 2016 after being cast as Jim Hopper in the hit sci-fi series following two decades in the entertainment industry.
Speaking to comedian Rob Brydon on his podcast Brydon &, the Black Widow actor shared: "It's crazy. This happened to me at 40, so for a long time I was walking around the streets just being that guy."
Gavin & Stacey star Brydon suggested that it's 'probably good' to reach celebrity status at an older age.
Harbour agreed and explained: "In my estimation, it's a good thing. Also, you don't take yourself that seriously, because you realise that, you know, you're pretty close to death."
57-year-old Brydon then went on to say that he believes his 50s have been the 'peak of the hill' in life.
"You can have a good time coming down that other side of the hill, but I think when you look back, you'd say 50 is probably the peak,” the Welsh actor shared.
Amid their conversation about fame, Harbour admitted that Stranger Things came at the right time in his life, revealing he had been struggling in the acting industry and was worried about making his rent.
"I was on a show that was particularly terrible that had one season, and I was waiting for it to get picked up for a second season, which I was hoping and praying [for] because I couldn't pay my rent," he said.
"At the time, Netflix was like House of Cards and Orange is the New Black and, you know, the series that were big, artistic ventures."
Convinced he wouldn’t get the part, he shared how he felt after he got the call: "I was like 39, I was like this isn't happening, I'm not getting the lead in this series anymore. And then they just called and were like 'you're our guy'...
"My heart exploded and I was just running down the street... it was an amazing day."
Previously, Harbour revealed he lost more than five-and-a-half stone ahead of the latest season of Stranger Things.
The second volume of the fourth series dropped on Netflix at the start of the month, and it's all a lot of people want to talk about at the moment.
Harbour's fan favourite, Jim Hopper, looks like a different fella in the new series and the actor said he had to lose a huge amount of weight to reflect the toll of prison life on his character.
"You know at the end of season three I was about 270 pounds, and then when we came into this season I was 190," he said on BBC Breakfast yesterday (5 July).
Featured Image Credit: Rob Brydon/YouTube/Alamy