County highway
CR 21 was added to the Suffolk County highway system on January 27, 1930, on various roads dating back to colonial times. A widening project took place along Yaphank Avenue during the 1970s but various realignments took place along Yaphank–Middle Island Road, and Rocky Point–Yaphank Road before then. Though CR 21 runs almost the entire breadth of Central Long Island, it does not intersect many major highways. Due to steep public opposition to highway improvements, there are multiple proposed and never-built highways that would have intersected this road: