The verdict against Salauddin Quader Chowdhury came as he stood in the dock in a packed courtroom amid tight security in the capital Dhaka.
Fearing a backlash from Bangladesh Nationalist Party supporters, authorities deployed paramilitary forces in south-eastern Chittagong district, where Chowdhury was elected to parliament six times.
Attorney General Mahbubey Alam said the tribunal convicted Chowdhury on nine of 23 charges, including four counts of genocide. Chowdhury was found guilty of aiding and ordering the killing of at least 200 people, mostly minority Hindus, during the war in Chittagong.
During the war, Chowdhury's father was a leader of the Muslim League party, which worked to prevent Bangladesh breaking away from Pakistan.
"I think this is a fair trial," Mr Alam said. "We are happy."
Chowdhury's wife Farhat Quader Chowdhury said her husband would appeal, adding: "We will do whatever we need to do to show the world that this is a farce."
Bangladesh said Pakistani soldiers, aided by local collaborators, killed three million people and raped 200,000 women during the nine-month war that ended in December 1971. Prime Minister Sheikh Hasina set up the tribunal in 2010 to punish the alleged collaborators.
The opposition, led by former prime minister Khaleda Zia, has criticised the trials as an attempt to weaken the Bangladesh Nationalist Party and its allies.