"I was trying to go away to get a ground-ball double play, " Leon said, "but I didn't execute very well the way I wanted to."
The last time the A's allowed an extra-inning grand slam at home was in 1989, when the Red Sox's Dwight Evans hit one off Dennis Eckersley.
Leon, who was recalled from Triple-A Nashville on Monday, is expected to have a longer-than-usual stint with the A's this time around, but it didn't get off to the start he was looking for.
The right-hander allowed singles to Caleb Joseph and Manny Machado to open the inning.