| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | R | 147,766 | 155,310 | 309,515 | |
| 2020 | D | 143,795 | 139,729 | 290,168 | |
| 2016 | R | 84,468 | 104,175 | 204,659 | |
| 2012 | R | 61,875 | 97,006 | 163,804 | |
| 2008 | R | 67,691 | 88,323 | 158,403 | |
| 2004 | R | 43,117 | 83,284 | 128,198 | |
| 2000 | R | 26,591 | 65,041 | 95,935 | |
| 1996 | R | 24,175 | 36,836 | 66,522 | |
| 1992 | R | 19,437 | 26,208 | 61,254 | |
| 1988 | R | 19,589 | 27,322 | 47,230 | |
| 1984 | R | 9,911 | 25,774 | 35,784 | |
| 1980 | R | 10,408 | 15,035 | 26,661 | |
| 1976 | D | 9,355 | 7,481 | 17,010 | |
| 1972 | R | 3,806 | 6,998 | 10,867 | |
| 1968 | D | 5,528 | 2,923 | 10,123 | |
| 1964 | D | 7,430 | 1,766 | 9,202 | |
| 1960 | D | 5,410 | 2,429 | 7,870 | |
| 1956 | D | 4,402 | 2,947 | 7,367 | |
| 1952 | D | 5,010 | 3,646 | 8,663 | |
| 1948 | D | 5,638 | 1,094 | 7,026 | |
| 1944 | D | 5,284 | 1,239 | 7,431 | |
| 1940 | D | 5,944 | 1,714 | 7,671 | |
| 1936 | D | 4,995 | 375 | 5,383 | |
| 1932 | D | 6,783 | 418 | 7,214 | |
| 1928 | D | 3,689 | 1,833 | 5,531 | |
| 1924 | D | 6,324 | 934 | 7,578 | |
| 1920 | D | 3,677 | 819 | 5,051 | |
| 1916 | D | 2,701 | 656 | 3,451 | |
| 1912 | D | 2,014 | 246 | 2,572 | |
| 1908 | — | — | — | — | |
| 1904 | — | — | — | — | |
| 1900 | — | — | — | — | |
| 1896 | — | — | — | — | |
| 1892 | — | — | — | — | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Williamson County's rapid suburban growth along the I-35 corridor has compressed a once-comfortable Republican margin to 2.5 points, making it one of the closer-watched bellwether counties in Central Texas.
The county's recent history is a story of close margins. The Democratic margin reached eighty-eight points in 1932; the Republican margin reached forty-four points in 1984. Most other elections have been decided by single-digit points.
Williamson County's demographics — a population of 672,688, a 60% non-Hispanic-white share, a median household income of $111,340 — situate the county close to national averages on several dimensions. The county's voting pattern over the last decade is most similar to that of Chelan County and Denton County.