Cantu's solo shot off Jeff Niemann (6-1) put the Marlins up 2-1 and Helms capped the four-run rally with homer with a two-run clout, giving Florida the rubber match of a three-game inter-league series.
Mike Stanton was 2-for-2 with a pair of RBIs and Dan Uggla chipped in with an RBI for the victors.
Chris Volstad (4-6) hurled a solid six innings, yielding only one run on four hits, walking four and striking out three in winning for the first time in seven starts.
Niemann was touched for five runs on nine hits over six innings.
Notable deaths of 2014 [PHOTOS]
UPI Sports Results for September 19, 2014