The special court in Islamabad announced the verdict on December 17 with a 2-1 majority.
Musharraf, who ruled Pakistan between 1999 and 2008, is the first military ruler to stand trial in Pakistan for overruling the constitution.
The ex-strongman is now living in exile in Dubai in the United Arab Emirates.
In 1999, Musharraf overthrew then-Prime Minister Nawaz Sharif in a bloodless military coup and served as president from 2001 to 2008.
He suspended the constitution and imposed emergency rule on November 3, 2007, before resigning in August 2008 to avoid possible impeachment.
The high treason charge has been pending against him since 2013.
Musharraf was allowed to leave the country for medical treatment in 2016.
A detailed verdict against Musharraf is to be issued later.
The military has ruled Pakistan for almost half of its existence since the country’s independence from Britain in 1947.