Their emotionally-charged, radio-friendly rock anthems like Radioactive and Sucker For Pain have won Grammy Awards, number one albums and a scarily loyal fanbase.
But behind closed doors, the band's frontman Dan Reynolds was struggling with severe depression.
Things came to a head on their last world tour. Playing 110 dates in 42 countries on five continents left the singer "numb".
"It came to a point where I didn't have an option," he tells the BBC. "It was lose my family and lose my life or seek help."
After the band's final concert of the tour in Amsterdam last February, the star told his bandmates he was taking a year off to tackle the condition, which has plagued him since childhood.
"When I was younger, in school, I would just let it pass, and ride out the valleys and the peaks and the ups and downs," he says.
"Then I tried medication and it was too scary for me, because I felt like it was going to change the music."
So last year, he "went to a therapist and sat down and basically faced it head on for the first time".
Having decided not to take antidepressants, his treatment took "a more holistic route" than before - changing his diet, cutting out sugar and practicing yoga and meditation.
The latter, in particular, "has been huge" for the singer, who says he has trouble "quieting" his brain.
Reynolds was supported through the last year by his wife, fellow musician Aja Volkman, and bandmate Wayne Sermon, whom he's known since childhood.
"We're brothers, so if he's not in a good place, then none of us are," says Sermon.
The guitarist admits everyone in the band has "demons that came out" after their ascent to fame - including his own issues with trust and "questioning people's motives".
"When you're under a microscope, it's going to magnify all the problems you had before," he says, "But I don't want to be to be down about it, because I feel like we've come out better people. All of us."
Those changes are the foundation of Imagine Dragons' new album Evolve.
The artwork shows a human form ascending from a place of eternal darkness to one of light and colour, while the music is inventive and vibrant - especially when compared to the claustrophobic onslaught of their last record.
"Smoke + Mirrors was super-angsty" admits Reynolds. "This album is a celebration of opening the blinds and the light shining through."
On the lead single, Believer, he writes about his personal transformation while acknowledging: "My life, my love, my drive, it came from pain."
"I was raised in a really conservative household and carried a heavy guilty conscience with me because of religion," he explains. "Letting go of that was really freeing and wonderful. But I've also learned to be happy for the things I learned from it and grateful for the love that I found.
"If it wasn't for all of those things working in the exact way they did, Imagine Dragons would cease to exist."
The seventh of nine children, Reynolds was raised in a strict Mormon household. Aged 19, he volunteered to become a missionary in Nebraska for two years, proselytizing for the church and helping drug addicts.
On his return, he enrolled to study music at the Mormon-run Brigham Young University in Utah, but dropped out to form Imagine Dragons with Sermon and two of his classmates from the prestigious Berklee College of Music in Boston.
In the beginning, they took any gig they could get, even opening for a mime at a nearby shopping centre. One regular booking at a local casino saw them playing six-hour sets three times a week, mixing original material with crowd-pleasing covers.
Their big break came in 2010 when roots rock band Train pulled out of a local festival. With just half an hour's notice, Imagine Dragons were promoted to the main stage and played for 26,000 people.
Within a year, they had signed to Interscope Records and started work on their debut album, Night Visions.
Like fellow Las Vegas rock band The Killers (who are managed by Reynolds' older brother, Robert), the group straddle the divide between rock and pop, embellishing their fist-pumping choruses with the sleek textures and throbbing rhythms of dance music.
It's no surprise, then, that they're one of the few rock acts to score commercial success at a time when pop and hip-hop dominate the charts.
"One of the reasons we called our band Imagine Dragons is so we had no boundaries," says Reynolds. "We created music that was all over the map because we didn't want people to expect power chords on my guitar or a simple pop song every time.
"We wanted the leash to be gone and for us to be able to run wild in the field."
The band's eager eclecticism has irritated purists ("pop deserves better, rock deserves better," sniffed one review) but it makes perfect sense in the streaming era, when fans flit between genres on a whim.
On Evolve, their sound has also become leaner - free from the sonic, as well as the emotional, baggage of their last record.
A song called Yesterday is particularly eye-opening, with Reynolds singing like a bluesman wandering through the dustbowl, over a gregarious piano stomp.
"We really reached into our roots of being inspired by the more eccentric songs from the Beatles and Queen," says Reynolds.
The band were propelled out of their comfort zone by recruiting an array of top-shelf producers, including Lorde's right-hand man Joel Little, and Swedish duo Mattman & Robin, who have worked for Britney Spears and Taylor Swift.
They even got some help from Justin Tranter, one of pop's most in-demand writers, whose credits include Justin Bieber's Sorry and DNCE's Cake by the Ocean.
"He understood my trepidation at working with another writer," says Reynolds.
"I've never done it before - but he knew that I had to write all my own lyrics and he was a wonderful sounding board, melodically."
The singer is excited for fans to hear the record ("it feels like Christmas is coming"), especially after the group's year-long hiatus.
"A lot of our fans really invest their life into this band," he says. "They come to every show, they get tattoos, their whole wall is lined in Imagine Dragons things.
"And I get it, because I was that kid, you know? That's what Nirvana was to me when I was a little kid. Kurt was everything to me.
"So I feel a responsibility to all of the people who invest their time into Imagine Dragons... an obligation to be honest and tell them what my heart says."
That's why he decided to write about his depression and to address it in interviews.
"I was really shy about speaking out," he says. "Humanity doesn't like to acknowledge their frailty. We like to only acknowledge our strengths. But I think it's important to talk about it."
"It's like the song Believer," adds Sermon. "Of course, every human wants to avoid pain at all cost - but the truth is, that's one time we really grow."
Imagine Dragons' third album, Evolve, is released on Friday 23 June.
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Over the last five years, Imagine Dragons have become one of the biggest bands on the planet.