The French outfit had gone behind in the first minute of the game, when striker Luka Jović had smashed in on the rebound following a block on the line.
Frankfurt's attackers were not needed in order to double their lead, thanks to the gaff from Brazilian defensive midfielder Luiz Gustavo.
The Europa League finalists from last season were passing the ball around the back, with the former Bayern Munich under pressure from the Frankfurt press.
Instead of passing it back to Yohann Pelé in the Marseille goal, Gustavo mis-hit his pass and saw it roll in to the net past the helpless goalkeeper.