Such a verdict is a first in a country with a history of army rule. The military reacted angrily, saying legal process "seems to have been ignored".
The general seized power in a 1999 coup and was president from 2001 to 2008.
The penalty is unlikely to be carried out. Gen Musharraf was allowed to leave Pakistan in 2016 and is in Dubai.
The high treason charge has been pending since 2013. It relates to Gen Musharraf's suspension of the constitution in 2007, when he declared an emergency in a move intended to extend his tenure.
