A jury ordered the Montgomery Ward department store chain yesterday to pay more than a half-million dollars to a former appliance saleswoman at its Gaithersburg branch as damages growing out of her mistaken arrest there for shoplifting 14 months ago.

The Montgomery County Circuit Court jury in Rockville awarded Joan Nieto, 50, of Mount Airy, Md., $500,000 in punitive damages plus $34,500 to compensate for lost wages, hardship and legal fees. In addition, the security guard who arrested her was ordered to pay $2,000 in damages.

The 12 jurors had deliberated since Monday afternoon.

The suit grew out of Nieto's arrest in the store by security officer William McCauley on Dec. 23, 1979. She and her attorney, Al Adelman, gave this account:

The three-year employe of the store had finished work and was doing some last-minute Christmas shopping. Aware that she would need special clearance to use her credit card because it was close to its limit, she had gathered merchandise worth $51, including berets, record albums and a child's game, and was carrying them toward the credit department to pay for them.

In the automotive department on the ground floor, at a point about 50 feet from the nearest exit, she was stopped by the store's security chief, William McCauley, at about 7:30 p.m. and arrested. He took her to his office, asked her to sign a confession, and later called the police. Nieto was charged with theft and three days later was informed that she was fired.

Last March 14, the state's charges against her were dropped on the advice of Montgomery Ward, Adelman said. Shortly afterward, Nieto filed suit seeking damages and alleging false arrest, defamation and malicious prosecution.

Montgomery Ward never invited Nieto to return to her job, her attorney said. In the interim, she has held part-time secretarial jobs but has not sought another sales position because of difficulties in obtaining a reference. s

Following the jury's verdict, Nieto said: "Justice has been done. I think I've been vindicated." She expressed hope that the decision would make "security guards more cautious and make them listen to explanations."

A Montgomery Ward employe speaking for the Gaithersburg store's manager said the manager could have no comment. The store's attorneys were not available for comment.

The department store has 30 days to file an appeal with the Court of Special Appeals in Annapolis.