Grant of Millennial Money, who goes by his first name exclusively, went from broke to seven-figures in five years thanks to a side hustle that he turned into a lucrative consulting company. After landing a digital marketing job out of college, he quickly realized that his $50,000 salary "just wasn't going to be enough money. I wasn't going to be able to get ahead making this and saving 5% to 10% of my income," the now 31-year-old tells CNBC. Grant's solution was to start building websites in his spare time. "The first website I built was a $300 one for a law firm," he says. "About six months in, I sold a $50,000 side project." Within a year, he sold his first $100,000 project and decided to quit his full-time job to focus on growing his consulting business.

Grant of Millennial Money Courtesy of Millennial Money

His success didn't come without sacrifice. At the time, Grant was working 90 to 100 hours a week and "putting money first," which he says is the biggest financial mistake he's ever made. "Money is not everything, and it's not worth sacrificing your health, family, friends, or other experiences for it," he writes on his blog. "I have lost a few friends and strained other relationships because I've spent too much time staying late in the office or hustling on the weekends. Even though I truly believe that having money is freedom, money is really just a tool to make experiences in life possible."