On the wall at Mullen's Boston headquarters is a large, handsome black stallion. He symbolizes the shop's quest to be not the most popular agency in adland, but the one that surprises marketers and competitors to pull off big wins and unconventional campaigns.

If 2013 is any indication, though, the days Mullen can call itself a dark horse are numbered.

The agency beat out stiff competition to emerge in one of the highest-profile pitches of the year, luring American Honda Motor Co.'s Acura business from longtime incumbent RPA.

The win was proof that the 43-year-old agency, which in recent years has attracted entrepreneurial clients like Zappos, JetBlue, Google and iRobot, could be entrusted with a massive, highly complex account. It also provided momentum for Mullen in 2013 to hire a whopping 250 staffers, take on other accounts like American Greetings and Pep Boys, and swell by $32 million in revenue, which was good for 10% year-over-year growth. The agency hung its shingle in the hottest ad market in the U.S. right now, Los Angeles. And then there's the work, which pushed the boundaries of advertising with everything from a crowdsourced Vine video to a prank real-estate listing.

All told, it'll go down as the year in which Mullen, once merely a respectable regional player, solidified its position as a national powerhouse and shining jewel in parent company Interpublic's crown.

"We really started [2013] in the Acura pitch, going against agencies like 72andSunny and The Martin Agency ," said Mullen CEO Alex Leikikh. "For us that's a big part of the story." Mr. Leikikh, previously the agency's president, was tapped in December to write the next chapter in the agency's history, succeeding 30-year veteran and boss Joe Grimaldi.

Eleven months later, Mullen's ads have helped push the new Acura MDX to record sales, said Honda CMO Mike Accavitti. "You can argue about whether you like the spot, but not with the fact that [we] hit record sales," he told Ad Age. "Advertising made [people] aware of this new product and aware of the direction of the Acura brand."

One of the secrets to Mullen's success is positioning itself as a scrappy challenger with multifaceted capabilities -- creative, media, digital, PR -- and a knack for delivering clients some nontraditional thinking.
The strategy requires "throwing more creative talent at media briefs and more hybrid talent at more traditional briefs," said Mr. Leikikh. And it's no wonder the shop's most buzzed-about work incorporates paid, owned and earned media elements.

For Century 21, the shop placed a phony ad on Craigslist for the New Mexico home of "Breaking Bad" main character Walter White following the show's widely watched series finale. With a minimal budget, the marketer captured the conversation on Twitter and Facebook during a major TV event.

The shop was also behind a Zappos stunt that turned a luggage carousel at Houston's George Bush Intercontinental Airport into a Wheel of Fortune-style game. And for NatGeo, the agency created a "Killing Lincoln Conspiracy" site to promote docudrama "Killing Lincoln." The site, showcasing content from Library of Congress and the National Archives, immersed users in 1865 and the world of Wilkes Booth.

A big "aha" moment in 2013 was seeing how much the media group, led by John Moore, could contribute to the agency's overall success. "When there's a creative review, half the work is from media," Mr. Leikikh said. Mullen also says it is committed to "smashing together" people and eschewing the departments that most agencies have.

"We're not huge believers in formalizing anything and we're not formal and disciplined folks when it comes to creating departments," Mr. Leikikh said. "That's the antithesis of taking the best brains and smashing then together."

It's a far cry from Mullen's days sprawled out in a big mansion in Wenham, Mass. Today staffers sit elbow-to-elbow in downtown Boston. The vibe is intense and working there isn't for the faint of heart.

Mr. Leikikh says he's hell-bent on fostering a culture where tenacious, aggressive attitudes are non-negotiable, which might explain Mullen's 20% turnover rate despite so much growth. "When you're doing this volume of hires in any given year you're going to make mistakes," he said. "We do have high demands."

That culture is one worth preserving, according to Mr. Accavitti. "I feel as if we're working with a smaller shop. Their culture comes from the similar concept [at Acura] that we're products of working-class environments and had to work and scrape to get by and advance ourselves. I like that kind of a culture."