Montano's body was found in the elevator shaft of an apartment building near where he was living in the capital's Condesa neighborhood. The circumstances of his death were being investigated by Mexican authorities.
During his time in the bureau, Montano covered stories including the saga of nine young elephants from Namibia who wound up on an animal reserve in Mexico's Puebla state, and the shooting of three federal policemen at the Mexico City airport.
He was not on assignment at the time of his death. The U.S. embassy is monitoring the course of the investigation.
Montano had planned to attend a master's degree program in journalism at the University of Barcelona in the fall.
"Armando was a smart, joyful, hardworking and talented young man," said Marjorie Miller, AP's Latin America editor based in Mexico City.