The most predominant denominations among residents in Harrison County who adhere to a religion are Southern Baptists (44.11%), United Methodists (10.77%) and Disciples of Christ (10.76%).

Harrison County, along with the rest of the state of Missouri, is represented in the U.S. Senate by Josh Hawley (R-Columbia) and Eric Schmitt (R-Clayton).

The last Democrat to win support from a majority of Harrison County voters was Lyndon Johnson in 1964.

Like most rural areas throughout northwest Missouri, voters in Harrison County generally adhere to socially and culturally conservative principles which tend to influence their Republican leanings.

On the Democratic side, former Vice President Joe Biden (D-Delaware) both won statewide and carried Harrison County by a wide margin.

Incumbent President Donald Trump (R-Florida) won both Harrison County and statewide by large margins.

On the Democratic side, former Secretary of State Hillary Clinton (D-New York) won statewide by a small margin, but Senator Bernie Sanders (I-Vermont) narrowly carried Harrison County.

In the 2012 Missouri Republican Presidential Primary, voters in Harrison County supported former U.S.

Senator Rick Santorum (R-Pennsylvania), who finished first in the state at large, but ultimately lost the nomination to former Governor Mitt Romney (R-Massachusetts).

Incumbent President Barack Obama easily won the Missouri Democratic Primary and renomination.

In 2008, the Missouri Republican Presidential Primary was closely contested, with Senator John McCain (R-Arizona) prevailing and eventually winning the nomination.

Then-Senator Hillary Clinton (D-New York) received more votes than any candidate from either party in Harrison County during the 2008 presidential primary.

Despite initial reports that Clinton had won Missouri, Barack Obama (D-Illinois), also a Senator at the time, narrowly defeated her statewide and later became that year's Democratic nominee, going on to win the presidency.