| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 183,451 | 154,247 | 346,860 | |
| 2020 | D | 184,259 | 155,631 | 347,214 | |
| 2016 | R | 129,852 | 137,490 | 294,208 | |
| 2012 | R | 110,526 | 158,401 | 275,074 | |
| 2008 | R | 127,091 | 152,627 | 283,432 | |
| 2004 | R | 97,866 | 158,103 | 258,687 | |
| 2000 | R | 79,118 | 129,965 | 217,536 | |
| 1996 | R | 68,129 | 110,368 | 190,894 | |
| 1992 | R | 59,573 | 85,418 | 194,866 | |
| 1988 | R | 55,183 | 95,591 | 152,199 | |
| 1984 | R | 38,019 | 101,987 | 140,882 | |
| 1980 | R | 33,210 | 78,048 | 123,983 | |
| 1976 | R | 35,605 | 75,798 | 114,142 | |
| 1972 | R | 24,324 | 76,161 | 102,727 | |
| 1968 | R | 26,034 | 55,060 | 87,912 | |
| 1964 | R | 31,213 | 37,672 | 69,179 | |
| 1960 | R | 21,914 | 43,026 | 65,033 | |
| 1956 | R | 14,185 | 35,511 | 49,733 | |
| 1952 | R | 10,990 | 29,103 | 40,163 | |
| 1948 | R | 8,982 | 14,191 | 23,378 | |
| 1944 | R | 5,771 | 11,951 | 17,773 | |
| 1940 | R | 5,770 | 10,326 | 16,142 | |
| 1936 | R | 6,108 | 8,399 | 14,554 | |
| 1932 | R | 6,485 | 6,487 | 13,096 | |
| 1928 | R | 3,373 | 8,185 | 11,627 | |
| 1924 | R | 2,519 | 6,102 | 9,224 | |
| 1920 | R | 2,303 | 4,325 | 6,729 | |
| 1916 | D | 3,928 | 3,767 | 7,900 | |
| 1912 | D | 1,837 | 834 | 4,390 | |
| 1908 | R | 2,091 | 2,313 | 4,520 | |
| 1904 | R | 1,373 | 2,573 | 4,213 | |
| 1900 | R | 2,171 | 2,393 | 4,639 | |
| 1896 | D | 2,462 | 2,313 | 4,837 | |
| 1892 | R | 0 | 1,932 | 4,101 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Johnson County anchors the Kansas City suburban corridor and has trended blue across three consecutive presidential cycles, making it a bellwether for college-educated suburban realignment in a traditionally Republican state.
The Democratic margin in Johnson County has rarely exceeded twenty-three points in modern history; the Republican margin has rarely exceeded fifty points. 2024 delivered the county to the Democratic candidate by eight points.
Johnson County's demographics resemble the country more than they resemble most counties. A 79% non-Hispanic-white share, a 5% poverty rate, and a median household income of $109,208 — all within the broad national range. The county's voting pattern over the last decade is most similar to that of James City County and Kent County.