The result means that Herefordshire end up in fourth position of the five teams while Cornwall finish at the foot of the table and return to Division 2.
Cornwall, realistically needing a victory to stay up, won the toss and elected to field first.
But they were defied for more than 47 overs by Herefordshire's opening pair of Ben Chapman-Lilley (85) and Cyrus Shafi (89) who posted 130 for the first wicket before Shafi was caught at slip for his highest championship score.
There was a rare failure for skipper Matt Pardoe, who was caught behind for nine, while Ollie Walker miscued to mid-off for 20.
With Harry Sockett given out leg-before for four, at 186-4 Herefordshire were still well short of maximum batting points.
But Luke Tulacz, who has been a consistent performer this season in all formats, steadied the innings in a half-century partnership with Luke Powell (24).
Tulacz, who finished unbeaten on 75 with nine fours and a six, guided Herefordshire to their fourth batting points, before Pardoe declared on 328-9.
In the nine overs which remained on day two, Herefordshire claimed the wicket of opener Tom Orpe, smartly caught in the slips by Sockett off Sam Keeling Wright (1-15).
But heavy overnight rain on Monday night meant there was, in the end, no prospect of the home side adding to their overnight 25-1.
The result of this match and the drawn game at Sidmouth between Devon and Oxfordshire mean that Devon, the only side to win a match, take the title, while Cornwall, the only side to lose one, are relegated.