Democrat Mary Margaret Whipple took an early lead last night over Arlington County Board Chairman Stephen H. Detwiler, a Republican-backed independent, in a race that will determine control of theboard.

A victory for Whipple, who is also backed by the nonpartisan Arlingtonians for a Better County (ABC), would give the Democrats a majority on the five-member board until at least 1985. The Republicans have maintained control of the board since 1978 when Detwiler won his first four-year term.

If Detwiler wins reelection, he and the board's other two GOP-endorsed independents, Walter L. Frankland and Dorothy T. Grotos, would retain control of the board for at least another year. Frankland and Grotos come up for reelection next fall.

County voters also appeared in early returns to be rejecting a proposed redevelopment and housing authority.

County Republicans hoped Detwiler, a 39-year-old savings and loan executive, would reverse the trend of the last two elections when GOP-supported board candidates were soundly defeated by Democrat John G. Milliken and Democrat-backed independent Ellen M. Bozman. Because of those lop-sided victories, Detwiler said he shelved plans to retire from public office since control of the five-member board would be at stake this year.

Whipple, 42, a former County School Board chairman who ran unsuccessfully for the County Board in 1979, accused Detwiler and his GOP board colleagues of "back-room" dealings in several instances. She was particularly critical of the Republicans' handling of the firing of former County Manger Vernon Ford, the attempt to set school spending guidelines at a session that was opened only after citizen and press complaints, and the announced intention to appoint a School Board member before the deadline for applying for the post had expired.

Detwiler countered that all his actions had been proper and accused Whipple of raising old questions in an effort to mask the major issues of the campaign. He cited the Republicans' slashing of the real estate tax rate, increased citizen participation in county affairs through a variety of advisory boards and stiffened academic requirements for high school graduation under a GOP-controlled school board.

The two candidates clashed often on economic development issues. Detwiler argued that there has been orderly, responsible development along the county's two Metro subway lines under the Republicans. Whipple countered that Detwiler and his colleagues too often conceded to developers before considering the impact of their proposals on surrounding neighborhoods.