Felix Doubront (7-3) gave up just three hits and struck out nine in seven innings for the victory, Boston's second in a row over the Marlins.
The solo homer from Ortiz came in the fourth and gave Boston a 2-0 lead. He added a sacrifice fly during a two-run sixth and the Red Sox then expanded on a 4-2 advantage by sending 11 men to the plate in the eighth.
The first six batters reached base in that inning, accounting for three runs, and when Jarrod Saltalamacchia made the first out of the inning he did so with a sacrifice fly that drove in Adrian Gonzalez. Nick Punto finished off the outburst with a two-run single.
Ricky Nolasco (6-5) absorbed the loss, surrendering four runs on five hits in six innings.