Hornets keeper Lewis Carey was in inspired form in the first half, making a series of superb stops to keep the game goalless at half-time.
But Dec Rose, Malachi Linton and Jaiden Bartolo struck in the second half as the Stags completed a league double over Horsham, who failed to fire in Hampshire.
Defeat sees the Hornets drop to 14th in the table going into Tuesday evening’s home clash with second-placed Hornchurch.
See McCartney Martin’s photos from Horsham’s loss at Totton on this page and the ones linked, and get all the latest Hornets news in the West Sussex County Times, out this Thursday.
9. Flat Horsham FC beaten at AFC Totton - the match in pictures
Second half goals condemned Horsham FC to a 3-0 National League South loss at AFC Totton. Photo: McCartney Martin
10. Flat Horsham FC beaten at AFC Totton - the match in pictures
Second half goals condemned Horsham FC to a 3-0 National League South loss at AFC Totton. Photo: McCartney Martin
11. Flat Horsham FC beaten at AFC Totton - the match in pictures
Second half goals condemned Horsham FC to a 3-0 National League South loss at AFC Totton. Photo: McCartney Martin
12. Flat Horsham FC beaten at AFC Totton - the match in pictures
Second half goals condemned Horsham FC to a 3-0 National League South loss at AFC Totton. Photo: McCartney Martin