Personal finance startup NerdWallet has attracted more big-name backing from the world of traditional banking.

Simon Williams, a former top Citigroup executive and an early investor in Lending Club and OnDeck Capital, has invested $5 million in the San Francisco startup. That expands the size of NerdWallet's Series A financing, first announced in May, to $69 million.

The six-year-old startup, which reviews and sells credit cards and other financial products, is now valued at about $510 million. And dry as its description may sound, NerdWallet is on a bit of a hot streak this year.

Founded in 2009 by childhood friends Tim Chen and Jake Gibson, NerdWallet has decidedly humble roots: CEO Chen, laid off by a hedge fund during the financial crisis, started it with $800. Now NerdWallet is rapidly expanding headcount (from 149 at the beginning of the year to 270 currently) and office space, renovating an additional two floors in a building around the corner from its downtown San Francisco headquarters.

The federal government also just recognized Chen for his prominence in the rapidly-changing consumer financial industry: in September, the Consumer Financial Protection Bureau named him to its consumer advisory board, along with Affirm CEO and PayPal co-founder Max Levchin.

NerdWallet competes with some older, established websites with similar business models. Like Bankrate and Credit Karma, it lists credit cards and other bank products on its site. NerdWallet often (but not always) earns referral fees when one of its three million monthly users clicks through and signs up for a new bank account or cash-back card

Where Chen is trying to set NerdWallet apart is in scope and transparency. The company now offers to provide "truth for all of life's financial decisions," and says the payments it gets from banks do not affect its reviews of their products. The website also has a fast-growing editorial staff, publishing articles and clear advice about the personal finance industry, which often makes it easy to feel deluged by fine print and jargon.

Williams says he was attracted to the company's mission of "helping consumers understand ... the trade-offs and ... the attributes of" any credit card, bank account, or insurance they're shopping for on NerdWallet's marketplace-style website.

"It is very much more thoughtful around" telling each individual that "these are the kind of products you should be using," he said. "Not only should fintech be something that's disruptive; it should be something that has very clear benefits to the users."

Williams is the latest veteran of the traditional financial services industry to bet on NerdWallet. The company's advisers already include former Citigroup CEO Vikram Pandit and former American Express CEO James Robinson III. Its investors include Institutional Venture Partners, which led its Series A round, and RRE Ventures.

Williams spent the better part of a decade running several consumer banking operations for Citigroup, and last year finished a three-year stint overseeing HSBC's wealth management operations. But he was also early to spot the potential for the currently exploding generation of financial startups.

In 2007 he invested in small-business lender OnDeck, and in 2010 he joined the board of Lending Club (to which he returned as a director last year). Those companies went public in December, and while both have struggled since, their closely watched IPOs indisputably signaled the fintech sector's growing clout. 

That influence was recognized in September by the CFPB. Chen told me at the time that he hoped to make the regulator more recognizable outside of Washington and Wall Street, especially among the Millennials that NerdWallet largely markets itself to.

"Sometimes we joke internally that the CFPB is the public version of what we do," he said. "I would love to help make them more successful."