Carolina (8-1) jumped on St. John's starter Jeff Belge early thanks to RBI singles from Zack Gahagan and Ladowski. The Tar Heels added two more in the third when Gahagan stole home on a double steal with Kyle Datres before Cody Roberts singled home Datres.
But that would be it for the hosts offensively as Sean Mooney and Joe LaSorsa (1-0) combined to retire 16 of the last 17 batters they faced.
Despite that, UNC led 4-2 before Berardi's two-run shot off Rodney Hutchison Jr. tied the game in the seventh. Austin Bergner (0-1) replaced Hutchison and immediately got a line-drive double play to escape further trouble, but Donadio led off the eighth with a solo shot to right center.
LaSorsa worked a 1-2-3 ninth to seal an eighth straight win for St. John's, which is off to its best start since 1981.
Carolina continues its homestand Friday with the first of three against Long Beach State at 3 p.m.