Thomas S. Condit, president and chief executive officer of D.C. National Bank since 1978, has resigned unexpectedly to pursue personal business interests.

Robert P. Pincus, 36, currently executive vice president of D.C. National, is expected to be nominated as Condit's successor at a March 25 organizational meeting of the bank's directors. Pincus has been with D.C. National for 11 years.

Condit, 41, will continue as a director of the bank's holding company, D.C. National Bancorp Inc. He has agreed also to develop and assist in the implementation of a strategic plan for the holding company.

Although formal plans have yet to be developed, D.C. National has already indicated that this year it expects to explore several business areas, including investments in a small business investment corporation (SBIC), a mortgage company, discount brokerage services and interest futures. New services developed in those and other areas will be provided through affiliates of the holding company.

After six years as a senior executive with D.C. National, Condit said he has decided to explore other business opportunities that will permit him to become an owner instead of a manager.

Condit said he decided as early as last September to resign as president but that it has taken him this long to "put all the pieces together."

John Safer, chairman of the corporation and the bank, said Condit has played a vital role in the bank's success.

"Right now, the bank is in as good a shape as ever from a management standpoint, earnings and liquidity," Condit said. "We made more money last year than the bank did in the first 15 years."

Under Condit, D.C. National has pursued a strategy of providing banking services specially suited to local business needs.

"In the six years that Bob Pincus and I have worked together, we've made $200 million in loans and we've lost less than a million" in unpaid loans, Condit recalled.

Pincus said D.C. National took in $33 million in money market funds last year, more than bank officials had projected. The influx gives the bank a new source of funds "which will take care of our loan demand" this year, Condit added.

D.C. National opened in 1962. Although small by comparison with some other District banks, its earnings growth and other key indicators, including return on assets and return on equity, have put D.C. National among leading banks in its peer group in the Washington market.

Condit and Pincus, who share the same management philosophy, despite different styles, said they are optimistic that the bank will continue to operate at that level.

D.C. National finished 1982 with income of $2.3 million before securities transactions, marking the sixth consecutive year in which it achieved record earnings. Net income for the year decreased, however, from $2 million ($9.78 a share) to $1.9 million ($9.32). Bank officials noted that operating income was higher last year despite increased allowances for loan losses, several unanticipated loan losses, formation of the holding company and inflationary pressures in general.