Arlington County Board Vice Chairman John G. Milliken was the only Northern Virginia Democrat to win a contested race in yesterday's elections as he resoundingly defeated challenger Peter J. Espada, a Republican-backed independent, to win a second four-year term.

Milliken, who appeared to be the highest vote getter on the ballot in Arlington, captured a margin of better than 2 to 1 over newcomer Espada, who entered the race last June after other Republican activists rejected party leaders' requests to run against the popular Democrat.

Milliken's victory assures that Democrats, who now hold four of the five board seats, will retain their majority at least through 1986. Michael E. Brunner is the board's only Republican member.

"I think it's a positive statement about the kind of government we have locally," Milliken, 39, said of his victory last night. "I think that people believed services were good, taxes were relatively low and people voted according to that."

Espada, 33, an economist with the General Accounting Office, tried to link Milliken, a lawyer with Walter Mondale's Washington law firm, to the Democratic presidential nominee and to portray Milliken as a free-spending liberal beholden to special interests.

Milliken, who also is chairman of the Metro transit board this year, campaigned on his record, pointing to votes he said helped control development, provided services to meet the needs of the county's changing population and allotted more funds for schools.

The campaign was generally low-key, but marked by what Milliken and other Democrats complained was a series of misrepresentations of his record. In his campaign literature, Espada implied inaccurately, for example, that Milliken had voted for a temporary shelter for homeless men that the neighborhood opposed.

He also wrongly accused the Democrat of failing to vote against a higher real estate tax rate when Milliken voted with the rest of the board to cut it two cents to 97 cents per $100 assessed value, the area's lowest rate.

But the charge that Milliken said most annoyed him was Espada's accusation last week that the board's Democrats adopted a revised equal employment policy that would force the county and the School Board to recruit homosexuals. The policy, Milliken said, merely says the county cannot discriminate against a prospective employe on the basis of "sexual orientation," and does not require recruitment of homosexuals.

Milliken's victory represents the first time in almost 20 years that the board has had a member from South Arlington. He and his wife recently moved there.