Across much of Northern Virginia, Tuesday’s election reaffirmed the public’s support of incumbents in many local races — with the notable exception of Loudoun County, where residents bucked the trend and ousted several long-serving county officials.

In Loudoun, a predictably unimpressive voter turnout of roughly 30 percent was enough to ensure dramatic changes in county leadership: All nine seats on the county Board of Supervisors were claimed by Republican candidates, with just two incumbents — Chairman Scott K. York (R-At Large) and Supervisor Eugene A. Delgaudio (R-Sterling) — among the victors.

Loudoun voters also elected a new sheriff for the first time in 16 years, ousting four-term incumbent Stephen O. Simpson (I) in favor of Mike Chapman (R), who beat Simpson with nearly 54 percent of the vote, according to unofficial election results. Incumbent Commonwealth’s Attorney James E. Plowman (R) held his seat, defeating Democrat Jennifer T. Wexton by a relatively narrow margin, while the School Board saw a mixed slate of mostly new candidates elected to the dais, including several endorsed by school-focused local political action committees that formed this year to sharpen public attention on the contests.

York said he was looking forward to working with the new board members but emphasized the need to handle their leadership with care.

“It’s our time to govern, but we have to be cautious of how we go through it,” he said. “If we don’t do it right, we’ll be out in four years.”

In the upcoming term, the board intends to focus on expanding the county’s commercial tax base and solving transportation troubles, York said.

In Fairfax County, voters reelected all the incumbents of the county Board of Supervisors. In the tightest and most closely watched battle, John C. Cook — a Republican who won the swing district of Braddock in a 2009 special election after more than 20 years of Democratic control — claimed a narrow victory over Democratic challenger Janet S. Oleszek, according to unofficial results.

Board Chairman Sharon Bulova (D), along with fellow Democrats in the board’s 7 to 3 majority, had urged voters to reelect the team of leaders that guided the county through the worst recession in generations with limited cuts in services and relatively steady tax rates.

“I think we demonstrated that Fairfax County and our board has been on a positive, steady course, and what we heard from the voters last night was an affirmation,” Bulova said Wednesday.

Incumbent supervisors also fared well in Prince William County, where Chairman Corey A. Stewart (R-At Large) easily defeated two opponents, taking 58 percent of the vote, according to unofficial results. Stewart said he was particularly pleased to have won traditionally Democratic precincts in the eastern part of the county.

“Never in my wildest dreams did I think I would win by 30 percent,” Stewart said. “If there’s an underlying theme here, in Prince William people are voting on local issues and are very happy with the direction of the county. “

Peter Candland, the Republican nominee in Gainesville, replaced outgoing Supervisor John T. Stirrup Jr. (R). Martin E. Nohe (R-Coles) took 69 percent of the vote over challenger Anthony D. Arnold (D), and Democratic incumbent Frank J. Principi (Woodbridge) won 61 percent of the vote over opponent Chris Royse (R), according to unofficial results.

Incumbent Prince William Sheriff Glendell Hill (R) beat independent challenger Michael Messier, while longtime Commonwealth’s Attorney Paul B. Ebert (D), along with Supervisors Maureen S. Caddigan (R-Dumfries), Michael C. May (R-Occoquan), John D. Jenkins (D-Neabsco) and W.S. “Wally” Covington III (R-Brentsville), ran unopposed.

In Arlington County, the soon-to-be vacant seat of Barbara A. Favola (D) — the County Board member who won the race for state Senate District 31 — will prompt a special election sometime in the first half of next year. With just one year left on her term, Favola’s replacement would face another election within months, next November.

Favola said she would serve through Dec. 30, which means the special election would probably be in April in order to abide by specific rules that apply to special elections, said Mike Lieberman, chairman of the Arlington County Democratic Committee. Party members will hold a caucus in early February to endorse a candidate, he said.

“Plenty of people are chumming the waters” and considering a run at the seat, Lieberman said. At least two filed their intentions at the elections office, and another planned to do so Thursday, he said.

Staff writers Patricia Sullivan, Jeremy Borden and Frederick Kunkle contributed to this report.