Democrat John G. Milliken, a former aide to incumbent 10th District Democratic Rep. Joseph L. Fisher, was elected yesterday to a seat on the Arlington County Board, defeating Republican-backed independent S. J. (Sim) Pace.

Unofficial tallies showed the 35-year-old Milliken winning with 29,965 votes, or 54 percent, to 25,917, or 46 percent, for Pace.

Milliken, the only Northern Virginia Democrat to win election yesterday, campaigned heavily with Fisher, his political mentor, who lost his bid for a fourth term to Republican challenger Frank Wolf.

"We've never been a straight-ticket county," said Milliken last night. "A lot of people told me they were going to vote for me and Ronald Reagan." Milliken professed to be baffled by Fisher's defeat.

Pace could not be reached for comment.

Milliken's victory may mark the demise of a once-powerful force in Arlington politics, the nonpartisan Arlingtonians for a Better County organization, which played a secondary role in Milliken's campaign. Although he was endorsed by the ABC, which virtually dominated county politics since its founding in 1955, Milliken ran as a Democrat, breaking a 20-year county tradition of candidates running as independents with major party endorsements.

Two years ago county Republicans began labeling the ABC as the party of high property taxes and big budgets. They went on to capture control of enough seats on the five-member board to ensure Republican dominiation through 1983.

Although Pace frequently scored Milliken's ABC endorsement, he failed to win sufficient name recognition and had difficulty raising funds. A vice president of Blue Cross of the District of Columbia and a former Marine corps captain, Pace managed to raise about $20,000 to Milliken's $40,000.

A Washington lawyer and former chairman of the Arlington Democratic Committee, Milliken stressed his long involvement in civic affairs and his ties to Fisher, who carried Arlington last night in his losing race with Wolf.

Milliken, who will succeed outgoing Democratic board member John W. Purdy, found that Pace, a political newcomer plucked from political obscurity by the Republican trio that controls the board, agreed with him on most issues during the campaign.

Although Republicans, led by board chairman Walter L. Frankland Jr., have made large cuts in the property tax rate, Milliken reminded voters that they are paying higher taxes because of skyrocketing assessments and higher trash collection and water-sewer fees voted last year by the Republican-dominated board.

Development emerged as the other major issue in the low-key campaign. Both men said they were concerned about the proliferation of high-rise buildings surrounding Arlington's 10 subway stops. Purdy, although endorsed by the Democrats, often voted with Republicans Frankland and Stephen H. Detwiler on development questions. Some observers expect Milliken to join Republican Dorothy T. Grotos and Democrat Ellen M. Bozman in frequently opposing such development.