The county is named for John Campbell, Fourth Earl of Loudoun and governor general of Virginia from 1756 to 1759.

It was also rich in agriculture, and the county's contributions of grain to George Washington's Continental Army earned it the nickname "Breadbasket of the Revolution.

[10] U.S. president James Monroe treated Oak Hill Plantation as a primary residence from 1823 until his death on July 4, 1831.

[13] During World War I, Loudoun County was a major breadbasket for supplying provisions to soldiers in Europe.

Loudoun farmers implemented new agricultural innovations such as vaccination of livestock, seed inoculations and ensilage.

However, this has changed in recent years with Democrats winning Loudoun in all statewide campaigns after Republicans narrowly carried it in 2014.

As of the 2023 elections, Democrats hold a 7 to 2 majority on the Board of Supervisors and a 6 to 3 majority on the School Board, but Republicans hold all five countywide elected constitutional offices (Clerk of the Circuit Court, Commissioner of the Revenue, Commonwealth's Attorney, Sheriff, and Treasurer).

This makes Loudoun County a reliable state bellwether, having voted for every statewide presidential election winner since 1932.

The county's official motto, I Byde My Time, is borrowed from the coat of arms of the Earl of Loudoun.

[1][15] In the mid to late 20th century, as northerners gradually migrated to Southern suburbs, Loudoun County increasingly shifted to the Republican Party in supporting presidential candidates, and more local ones.

Before the 2008 election of Barack Obama, county voters had not supported a Democratic president since Lyndon B. Johnson in 1964.

In 2012 county voters again supported Obama, who took 51.5% of the vote, with Republican challenger Mitt Romney garnering 47%.

[16] Democrats carried the county again in the 2016 presidential election, when Loudoun swung heavily towards Hillary Clinton, giving her 55.1% to Donald Trump's 38.2%.

[18] Loudoun was one of ten counties that was won by McAuliffe, though it was his smallest margin of victory in Northern Virginia.

Juli Briskman had been fired from her job as a marketing analyst for a United States government and military subcontractor, after an AFP photo of her flipping off the motorcade of Donald Trump went viral on social media in 2017.

[34] In 2007 AOL announced it would move its headquarters from Loudoun County to New York City; it would continue to operate its Virginia offices.

[36] Loudoun County houses over 60 massive data centers, many of which correspond to Amazon Web Services’s (AWS) us-east-1 region.

[citation needed] The decline was likely highest among African Americans, who had worked in an agricultural economy that was becoming increasingly mechanized.

[citation needed] During the first half of the 20th century, African Americans moved out of rural areas to cities in the Great Migration.

[56] Almost 25% of Loudoun County residents were born outside of the United States, with the largest groups being from India, El Salvador, and Korea.

In 2011, Census survey data concluded that Loudoun County had the highest median income in the country at $119,134.

[61] The Federal Aviation Administration's Washington Air Route Traffic Control Center, the second-busiest facility of its kind in the nation, is located in Leesburg.

The public library system has won several awards, including 10th place for libraries serving a comparably sized population in 2006[69][70] Loudoun County is one of the counties in Virginia that elects to cover their employees in the Virginia Mortgage Assistance Program (VMAP).

The Silver Line of the Washington Metro provides service at the Dulles Airport, Loudoun Gateway, and Ashburn stations.

[72][73] Loudoun County schools recently ranked 11th in the United States in terms of educational achievement versus funds spent.

[citation needed] The population ranking of the following table is based on 2018 estimates by the United States Census Bureau.