The Irish fighter had been charged with strong-armed robbery and misdemeanour criminal mischief.
But on Monday state prosecutors said the alleged victim had gone abroad and stopped co-operating with police.
The incident took place on 11 March as McGregor, 30, left the Fontainebleau Miami Beach hotel.
According to a police report filed at the time, the fan was trying to take pictures when the fighter knocked it out of his hand, before stamping on it, picking it up and leaving with it.
The phone was valued at $1,000 (£760).
"The victim of the crime does not wish to return to the United States and prosecute this case," Prosecutor Khalil Madani told the Miami court on Monday.
McGregor had already settled a civil lawsuit with the victim out of court.