Armando Dippet served as Hogwarts headmaster before Albus Dumbledore.
Dippet appears in J.K. Rowling’s history of Hogwarts as the school’s headmaster during Tom Riddle’s student years. In Harry Potter and the Chamber of Secrets, Harry sees Dippet’s image in a memory connected with the opening of the Chamber and the death of a student.
Dumbledore was Transfiguration professor at Hogwarts before becoming headmaster. He succeeded Dippet after the earlier headmaster’s tenure ended, and Dumbledore remained in the position for much of Harry’s time at school.
Dippet is easy to confuse with Phineas Nigellus Black, another former headmaster whose portrait appears in the series. Minerva McGonagall becomes headmistress later, after the events of the main series, so she was not Dumbledore’s predecessor.