You don’t just become a business leader overnight. Most of America’s entrepreneurs and executives spent years climbing the corporate ladder. Long hours and hard work are the ingredients for success, and they started paying their dues at a young age.

In our regular First Job Friday feature on Information Station, we profile American success stories who paid their dues early on—and found great value in their humble beginnings.

This week, we’re focusing on Marissa Mayer, the former Google executive and current CEO of Yahoo. Long before she was leading top technology companies, Mayer was a teenage girl with no work experience and searching for her first summer job.

Mayer got her first break at age 16—when she was hired as a grocery clerk in her hometown of Wausau, WI. Brought on to work the cash register and bag groceries at a local store, she quickly began studying her more experienced coworkers. As Mayer described it, “[We] measured our items per minute rate during each shift” and her supervisor set a 40-item-per-minute goal, which motivated Mayer to scan fruits and vegetables like her life depended on it.

And scan she did. After a few weeks on the job, Mayer’s average was 38 to 41 items.

Meeting the 40-item goal taught Mayer several very important things: hard work, quick thinking, and efficiency above all else. In her words, she “saw firsthand the importance of a great work ethic” and “learned that speed mattered.” Mayer “also learned a lot about family economics, how people make trade-offs, and how people make decisions on something fundamental, like how to eat.”

So it comes as no surprise that Mayer’s applied these same life lessons to her corporate career, at Google—her first job out of college—and now Yahoo. At Google, for instance, she regularly put in 100-hour weeks overseeing the site’s aesthetics, and obsessed over thousands of pixels and color shades. Her attention to detail became the stuff of legend.

Mayer credits her first job, which instilled a sense of pride in the work at hand—even if it meant simply bagging apples and stacking shelves. As one of America’s most powerful CEOs, Mayer’s experience shows that low-skilled jobs are anything but low-important.