The disco ball was in use as early as 1897:
"The 1897, article of the "Electrical Worker" discusses the Third Annual Ball held on on January 6, 1897, at Roughaus Hall, Charlestown, and of the spectacular lighting display, which could be seen for miles around Boston. The letters N.B.E.W. were done with incandescent lamps of various colors on wire mesh over the ballroom, highlighted by a carbon arc lamp flashing on a mirrored ball. The affair was hosted by Brothers Flynn, Melville, Colvin, Smith, Ellsworth and Dacey. About 800 people enjoyed the spiked punch and melodies of Dunbars famous orchestra."
The mirrored ball - used without lights and music - is probably much older, with no known inventor.
They used to use mirror balls to repel witches in Europe.
But the Disco Ball ? Prior to the 70's it was simply known as a Mirror or Mirrored Ball.
Elements of disco ( discothèque) music first appeared on records as early as 1969 with Jerry Butler's "Only the Strong Survive"  and had its roots in Philly and New York Soul which themselves were a further evolution of the Motown sound. The Philly Sound which would later morph into Disco was typified by its lush orchestration and emphasis on percussion.