Mary Margaret Whipple, a former Arlington School Board chairman beginning her fourth year on the Arlington County Board, has been selected by the board's Democratic majority to succeed County Board Chairman John G. Milliken, Milliken said yesterday.

Whipple, a former legislative aide to state Sen. Edward M. Holland (D-Arlington) and a longtime community activist, will serve as chairman for one year beginning Jan. 1.

In the board's annual realignment yesterday, Albert C. Eisenberg, a former U.S. Senate staff member who was elected to the County Board in November 1983, was selected as vice chairman, Milliken said.

Ellen M. Bozman, reelected Nov. 5 after an easy victory over Richard J. Herbst, who ran as an independent with the endorsement of county Republican leaders, was sworn in to her fourth four-year term on the board.

The five-member board will officially elect the chairman and vice chairman of the County Board at a Jan. 1 organizational meeting. At that same meeting, the four Democratic members of the board also will state their agenda for 1986. Several noted yesterday that a priority would be the continuing effort to improve Arlington's "livability."

"The major cataclysmic land-use decisions are made," Milliken said. "My view is that over the next stretch of years we need to concentrate our attention on Arlington's livability" -- including transportation, availability of retail shopping, housing and parks.

Board members cited several actions in the past year aimed to preserve affordable rental housing and neighborhood amenities such as grocery stores.

In April, the board approved plans to maintain the 152-unit Westover Apartments as a rental complex through the use of federal subsidies rather than allowing the complex to be converted to condominiums.

Last week, Whipple noted, developers broke ground for a 12-story Rosslyn building that will include a Safeway store. "We found a way to ensure that a grocery store remained in a new development; we found a formula that worked," she said.

Bozman said the board will continue its efforts next year to provide for those on both ends of the age scale -- the elderly population, growing faster than any other age group in the county, and the increasing numbers of children who need day care.

In light of demographics showing the most rapid growth in the elderly population and the group of persons between 20 and 35, the county must concentrate on luring families, Whipple said.

"We seemed to be naturally attracting the young single professional and needed to make some special efforts to attract families of all types; that's something we will continue to push."

Bozman cautioned that imminent federal budget cuts will make the board's priorities more difficult to carry out. "This is the framework local government finds itself in. We are the ones who will have to make adjustments and pick up the tab," she said. "We're going to be dealing with uncertainty."