The district was created in the 1972 redistricting act (1971 Wisc. Act 304) which first established the numbered district system, replacing the previous system which allocated districts to specific counties.[5] The 66th district was drawn somewhat in line with the former Kenosha County 2nd district (Kenosha County excluding most of the city of Kenosha), but removed southeastern Kenosha County and replaced it with a number of neighboring towns in central Racine County and eastern Walworth County.
Other than the 1982 redistricting, which temporarily scrambled State Assembly districts, the boundaries of the 66th district remained relatively consistent for the next 40 years (1972–2011), comprising most of Kenosha County with a rotating set of rural towns from Racine and Walworth counties. That changed in the controversial 2011 redistricting plan (2011 Wisc. Act 43) which moved the district entirely into the city of Racine in Racine County, in area previously represented by the 61st and 62nd Assembly districts. The previous territory covered by the 66th district was then split between the 61st, 63rd, and 64th Assembly districts.[6][7] This was done as part of a broader gerrymander of the southeast Wisconsin districts to pack the majority of Racine and Kenosha Democratic votes into one state senate district.[8] This map was only slightly adjusted in the 2022 court-ordered redistricting plan. The 2024 redistricting plan unpacked Racine from the district, splitting Racine again between two Assembly districts. The 66th district kept the southern third of the city of Racine and expanded to comprise the neighboring villages in the southeast quadrant of Racine County.