Ujjwala Pawar, the public prosecutor, told the BBC's Zubair Ahmed in Mumbai that the death penalty was given as Mane's crime fell into the "rarest of rare" category.
Mane's death allowed the then-President of Guinea-Bissau, Koumba Yala, to align himself more closely with the Senegalese government and limit the military supplies.
During the trial, Mane's lawyer had pleaded that his client was "mentally imbalanced", but the judge ruled that he was of a "sane mind", our correspondent reports.