Katia Beauchamp talks about cosmetic products the way a tech CEO might talk about a smartphone's functionality. Details, like the salt content in hairspray or the amount of product that clings to the tip of a mascara brush, are critical, and the beauty-junkie consumers her company serves rival Apple users in their tendency to be both obsessive and vocal about their products.
Beauchamp, 30, has been immersed in the beauty world ever since launching Birchbox with Hayley Barna in 2010, a few months after the two graduated from Harvard Business School, and her ability to speak her customers' language is key to the company's success. (Barna and Beauchamp share the No. 31 spot on our 40 Under 40 list)
Birchbox pioneered a subscription sampling model: Members pay a small monthly fee and receive a package of five beauty samples that they can then buy on Birchbox's website in full size. It was an idea that took off, and within a year there was a clutch of cosmetic box-of-the-month competitors.
With $12 million in funding, Birchbox is dwarfed by other startups on our list. But it has quietly amassed some 400,000 subscribers, 200 employees, and revenue of more than $40 million. Growth is still surging, largely thanks to the acquisition of JolieBox, with a presence in France, the U.K., and Spain. Its men's business, now a year and a half old, is also boosting sales, and more than half of all subscribers purchase full-size products on Birchbox.com.
The co-CEO setup is unique, and the founders indeed divide responsibilities evenly: Barna focuses on operations, technology, and customer relationships, while Beauchamp woos new brands and partners with companies, and steers the brand's image in the critical beauty press.
As Birchbox's profile has grown, so has Beauchamp's. But her roots veer far from the sparkly beauty industry scene. Her parents divorced when she was 4, and she spent the bulk of her years in El Paso and her summers in Germany, where her father relocated, and Greece (her mother is Mexican; her father Greek). That meant traveling alone with her 7-year-old brother when she was just 5, but the dual life triggered her interest in global studies. In 2001 she headed to Vassar College; her first day on campus was also her first time visiting the East Coast.
The summer before her sophomore year, Beauchamp won a coveted internship at Estée Lauder, an experience she says was life-changing. She finagled a role working on the AV team so that she could sit in on executives' meetings, and she became fascinated with the beauty business, where marketers had close knowledge of customer behavior and preferences. When she and Barna began working on a business plan, Beauchamp scrapped the paperwork and cold-emailed the CEOs of cosmetics companies she admired. After several agreed to work with the then-unnamed Birchbox, the duo decided to launch in September 2010 without the help of outside investors.
In a few short years Beauchamp has also evolved as a leader. She decentralizes power among her team, allowing them to execute their own ideas to realize Birchbox's mission. "We're building a bridge over a ravine and putting the next brick down as we step," she says. One sample-size lip gloss at a time.
This story is from the October 07, 2013 issue of Fortune.