| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | R | 75,370 | 83,933 | 162,283 | |
| 2020 | R | 78,154 | 82,260 | 164,578 | |
| 2016 | R | 60,803 | 71,612 | 143,930 | |
| 2012 | R | 59,797 | 71,598 | 134,237 | |
| 2008 | D | 72,288 | 64,845 | 139,268 | |
| 2004 | R | 50,330 | 76,412 | 127,948 | |
| 2000 | R | 40,698 | 62,112 | 106,185 | |
| 1996 | R | 31,240 | 41,136 | 83,256 | |
| 1992 | R | 24,783 | 41,356 | 88,294 | |
| 1988 | R | 18,919 | 46,135 | 65,499 | |
| 1984 | R | 14,420 | 47,282 | 62,042 | |
| 1980 | R | 14,540 | 40,045 | 61,655 | |
| 1976 | R | 16,799 | 37,115 | 54,989 | |
| 1972 | R | 12,090 | 36,114 | 48,312 | |
| 1968 | R | 10,896 | 27,245 | 41,185 | |
| 1964 | R | 18,014 | 22,503 | 40,517 | |
| 1960 | R | 12,659 | 25,787 | 38,508 | |
| 1956 | R | 6,820 | 24,912 | 31,798 | |
| 1952 | R | 7,218 | 20,975 | 28,257 | |
| 1948 | R | 5,459 | 15,387 | 20,949 | |
| 1944 | R | 5,567 | 15,666 | 21,299 | |
| 1940 | R | 6,170 | 16,480 | 22,795 | |
| 1936 | R | 6,893 | 12,031 | 19,866 | |
| 1932 | R | 8,260 | 9,880 | 18,594 | |
| 1928 | R | 5,596 | 10,661 | 16,319 | |
| 1924 | R | 1,372 | 8,751 | 13,015 | |
| 1920 | R | 1,536 | 9,885 | 11,616 | |
| 1916 | R | 3,278 | 9,024 | 12,532 | |
| 1912 | R | 1,913 | 2,370 | 7,447 | |
| 1908 | R | 1,887 | 5,331 | 7,501 | |
| 1904 | R | 1,309 | 5,409 | 7,006 | |
| 1900 | R | 2,076 | 5,234 | 7,468 | |
| 1896 | R | 1,913 | 5,047 | 7,117 | |
| 1892 | R | 2,311 | 3,205 | 5,810 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
A fast-growing exurban county northwest of Chicago, McHenry shifted from R+20 margins a decade ago to R+5.3 in 2024, reflecting the broader realignment of college-educated suburban voters across the collar counties.
The Democratic margin in McHenry County has rarely exceeded five points in modern history; the Republican margin has rarely exceeded seventy-two points. 2024 delivered the county to the Republican candidate by five points.
McHenry County's demographics resemble the country more than they resemble most counties. A 80% non-Hispanic-white share, a 6% poverty rate, and a median household income of $104,802 — all within the broad national range. The county's voting pattern over the last decade is most similar to that of Kendall County and Seminole County.