On her first day as chief executive officer of Wex Inc., Melissa Smith said she plans to manage the company’s transition into an increasingly global enterprise, and help grow beyond its core business of vehicle fleet credit card processing.

Smith, 44, who assumed the CEO position and a seat on the company’s board on Thursday, said the South Portland-based company is expanding into handling corporate financial transactions in other industries, such as travel and health care, which, along with Wex’s expansion overseas, requires constant “tweaking.”

Smith, who has been with Wex for 16 years, was tapped last May to succeed Michael Dubyak as CEO of the company originally called Wright Express. Dubyak, who will be executive chairman of the board for a year before retiring, has been with the company for 28 years, the last 15 as president and CEO.

“The company changes every few years,” Smith said. “We have a lovely problem: We grow. It’s the best of all problems.”

Smith said Wex will maintain its commitment to Maine. Although just 600 of the company’s 1,400 employees are based in South Portland, its headquarters will remain and grow as the company does, she said.

“Our headquarters are here, and that will continue to expand,” she said.

Smith noted that WEX has expanded its overseas presence, reaching a deal in November to buy ExxonMobil’s fleet card business in Europe that will add 1 million cardholders in nine countries. Terms of the deal were not disclosed, but it is expected to bring in about $35 million in annual revenues, and will close in about a year.

It follows major purchases in Australia, Brazil and England, and Smith said it will be a challenge to keep everyone on the same page when it comes to company goals.

Smith said she and Dubyak share a commitment to growth and similar visions for corporate goals, but she characterized her management approach as somewhat different from his – less formal, for one thing. She also plans to use more technology to maintain personal connections, sometimes preferring to videoconference with employees elsewhere instead of sending an email.

“Stylistically, we’re different,” she said. “Neither one is better, but we come across differently.”

The company’s market capitalization is approaching $4 billion, and its stock price has risen nearly 25 percent over the past year. In the third quarter of 2013, adjusted net income rose 20 percent to $50.4 million from $42.0 million for the same period in the previous year, according to company filings.

Those results have also paid off for top executives. Dubyak’s total compensation in 2012 was nearly $2.5 million, including $1.3 million in stock awards. Smith took home nearly $1.3 million last year, including $529,000 in stock awards. Smith still holds more than 33,000 shares of Wex stock worth a total of more than $3 million.

Smith said her first day on the job with a new title was mentioned around the office repeatedly Thursday, but the seven-month transition made the move seem smooth to her.

“I was a little surprised when I came today and so many people were saying, ‘This is the day,’ ” she said. “It really happened gradually.”

Edward D. Murphy can be contacted at 791-6465 or at:

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