PORTLAND, Maine — The CEO and co-founder of one of Portland’s most successful tech startups has resigned.

David Stone, founding CEO of digital gift card company CashStar, a venture capital darling, told the Bangor Daily News on Thursday that he left two weeks ago based on a “mutual decision” with the company’s board of directors.

Silicon Valley entrepreneur Steven Boal, chairman of CashStar’s board, has taken over as interim CEO, according to the company’s website. Boal is CEO of Coupons.com Inc., which employs roughly 500 people and is headquartered in Mountain View, Calif. He has a track record of leading successful tech companies.

Stone, a former American Express executive, co-founded CashStar with Boal in late 2007.

The company provides “digital gifting” solutions and has built an impressive roster of clients, including Starbucks, Best Buy, Home Depot and Williams-Sonoma.

CashStar employs more than 80 people and has raised “north of $30 million” in venture capital and private investment, Stone said. Its most recent investment round, announced last July, raised $5 million. Its headquarters are on Middle Street in the city’s Old Port.

“Under David’s leadership, the company has seen amazing growth,” Gene Cornfield, CashStar’s vice president of marketing, said Thursday. “Now the company has gotten to point where David and the board agreed that it was time to move to the next stage of growth.”

Boal has “a lot more experience with companies at this stage,” Cornfield said.

The opportunity to have Boal lead the company “is absolutely a good thing,” Cornfield said. “It’s great for the company, and great for the rest of us on the management team to work with someone with Steven’s wealth of experience.”

However, Boal is not leaving Coupons.com to lead CashStar full time, Cornfield said.

“I don’t think we’re getting that lucky,” he said. “Our expectation is that we will have a new permanent CEO at some point.”

Calls to Boal at his office in California were not returned Thursday afternoon.

Stone, 56, said his passion is building early-stage tech companies for high growth. CashStar’s rapid success meant it was time for him to step aside, he said.

“It was a good time to turn it over to others who have [bigger companies] in their sweet spot,” Stone said.

He added that he’s proud of the company he built and grateful to the company’s board and investors for allowing him to build a tech company in Maine, which “certainly wasn’t easy.”

“[Maine] is not considered a high-tech hub, but CashStar is trying to change that,” said Stone, a Falmouth resident. “It has, to some degree, but there isn’t a lot of capital coming into the state.”

But building a tech company in Maine can be done, he said.

“We’ve trailblazed, as the expression goes, and I’m sure more will follow,” he said. “If I have anything to do with it, they will.”

When asked about his future plans, Stone said it’s time “to reboot and refresh.” The digital payment sector is “burning hot” right now, he said.

“Every day a new mobile payment company is coming out. I’m sure I’ll have plenty of opportunities,” he said.

Whether one of those new mobile payment companies will pop up in Maine under Stone’s leadership remains to be seen.

“No guarantees,” he said. “But I hope so.”

Whit Richardson is Business Editor at the Bangor Daily News. He blogs about Maine business, entrepreneurs and the economy.