| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 9,048 | 6,670 | 16,100 | |
| 2020 | D | 10,356 | 6,256 | 16,968 | |
| 2016 | D | 8,423 | 5,953 | 15,411 | |
| 2012 | D | 8,478 | 6,463 | 15,200 | |
| 2008 | D | 7,518 | 5,975 | 13,627 | |
| 2004 | R | 5,562 | 7,257 | 12,903 | |
| 2000 | R | 5,262 | 6,752 | 12,410 | |
| 1996 | R | 4,378 | 5,799 | 10,960 | |
| 1992 | R | 3,647 | 5,453 | 11,154 | |
| 1988 | R | 2,658 | 5,980 | 8,719 | |
| 1984 | R | 1,824 | 4,613 | 6,466 | |
| 1980 | R | 1,565 | 3,009 | 4,952 | |
| 1976 | R | 1,646 | 1,992 | 3,737 | |
| 1972 | R | 695 | 1,345 | 2,070 | |
| 1968 | — | — | — | — | |
| 1964 | — | — | — | — | |
| 1960 | — | — | — | — | |
| 1956 | — | — | — | — | |
| 1952 | — | — | — | — | |
| 1948 | — | — | — | — | |
| 1944 | — | — | — | — | |
| 1940 | — | — | — | — | |
| 1936 | — | — | — | — | |
| 1932 | — | — | — | — | |
| 1928 | — | — | — | — | |
| 1924 | — | — | — | — | |
| 1920 | — | — | — | — | |
| 1916 | — | — | — | — | |
| 1912 | — | — | — | — | |
| 1908 | — | — | — | — | |
| 1904 | — | — | — | — | |
| 1900 | — | — | — | — | |
| 1896 | — | — | — | — | |
| 1892 | — | — | — | — | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Manassas City, once a competitive Northern Virginia locality, has shifted toward double-digit Democratic margins as its Latino and immigrant populations grew substantially over the past two decades, making it a case study in suburban demographic transition.
The 2008 election was the turning point, when the Democratic candidate carried the county by eleven points. The 2024 margin was fifteen points, in line with the county's new direction.
The demographics suggest why. A population of 42,976, a 43% non-Hispanic-white share, and a median household income of $113,590 situate the county in the broader realignment patterns of recent cycles. The county's voting pattern over the last decade is most similar to that of Loudoun County and Hopewell city.