| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 12,063 | 11,630 | 24,513 | |
| 2020 | D | 12,765 | 11,610 | 25,171 | |
| 2016 | R | 9,341 | 10,107 | 21,974 | |
| 2012 | R | 8,977 | 11,507 | 21,101 | |
| 2008 | R | 10,495 | 12,111 | 22,997 | |
| 2004 | R | 7,908 | 12,672 | 20,911 | |
| 2000 | R | 6,188 | 10,672 | 18,253 | |
| 1996 | R | 6,746 | 11,113 | 19,607 | |
| 1992 | R | 7,933 | 8,394 | 21,797 | |
| 1988 | R | 7,283 | 9,507 | 17,007 | |
| 1984 | R | 5,975 | 11,308 | 17,458 | |
| 1980 | R | 5,224 | 8,904 | 16,818 | |
| 1976 | R | 6,540 | 9,518 | 16,568 | |
| 1972 | R | 5,333 | 11,120 | 16,809 | |
| 1968 | R | 4,258 | 8,296 | 13,345 | |
| 1964 | R | 5,597 | 6,396 | 12,137 | |
| 1960 | R | 3,482 | 9,068 | 12,600 | |
| 1956 | R | 2,784 | 9,385 | 12,213 | |
| 1952 | R | 2,352 | 9,799 | 12,201 | |
| 1948 | R | 4,052 | 9,227 | 13,567 | |
| 1944 | R | 2,659 | 6,511 | 9,244 | |
| 1940 | R | 3,293 | 7,420 | 10,818 | |
| 1936 | R | 4,104 | 6,077 | 10,280 | |
| 1932 | R | 4,101 | 5,337 | 9,765 | |
| 1928 | R | 1,791 | 6,592 | 8,461 | |
| 1924 | R | 1,646 | 5,455 | 7,790 | |
| 1920 | R | 1,610 | 4,875 | 6,626 | |
| 1916 | R | 2,637 | 3,320 | 6,343 | |
| 1912 | D | 1,170 | 425 | 3,815 | |
| 1908 | R | 1,289 | 2,276 | 3,688 | |
| 1904 | R | 523 | 2,251 | 2,991 | |
| 1900 | R | 1,279 | 2,119 | 3,435 | |
| 1896 | R | 1,443 | 1,890 | 3,383 | |
| 1892 | R | 0 | 1,427 | 3,046 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Home to Kansas State University, Riley County's electorate skews younger and more educated than surrounding Flint Hills counties, contributing to a rare leftward tilt in an otherwise deeply Republican stretch of the Great Plains.
The county's recent history is a story of close margins. The Democratic margin reached twenty points in 1912; the Republican margin reached sixty-one points in 1952. Most other elections have been decided by single-digit points.
Riley County's demographics — a population of 71,946, a 77% non-Hispanic-white share, a median household income of $61,098 — 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 Williamson County and Anchorage Municipality.