Kumar, who has been asked to surrender by the end of this month, will have to spend the rest of his life behind bars.
A veteran Indian politician was given a life sentence on Monday over anti-Sikh riots in 1984 that killed nearly 3,000 people following the assassination of then-premier Indira Gandhi.
The Delhi High Court found Sajjan Kumar, 73, guilty of instigating mobs during the mass killings triggered by the death of Gandhi at the hands of her Sikh bodyguards.
At the time Kumar was an MP with the then-ruling Congress party. He was acquitted in 2013 but the High Court reversed the sentence on appeal from federal investigators.
He was found guilty over a case involving the murder of five members of a Sikh family in New Delhi after key testimonies from eyewitnesses.
A two-judge bench convicted Kumar for criminal conspiracy, promoting enmity and acting against communal harmony, the Press Trust of India and other local media reported.
"It is important to assure the victims that despite the challenges truth will prevail," the court said, according to the NDTV news network. "The aftershock of those atrocities is still being felt."
The 1984 carnage erupted just hours after Gandhi was shot dead at her residence in New Delhi. It lasted three days with Sikhs raped and murdered, their homes and businesses torched.
Gandhi was killed over her decision to use military force to expel Sikh separatists from inside the Golden Temple — Sikhism's holiest shrine in the northern Indian city of Amritsar.
Critics accuse Congress of turning a blind eye to the killings and the role of leaders such as Kumar and Jagdish Tytler.
Last week, Congress named Kamal Nath as the chief minister of the central state of Madhya Pradesh despite allegations that he had led one of the mobs during the riots.
Kumar, Tytler and others have always denied any wrongdoing. Kumar last won a parliamentary election in 2004 but was forced to withdraw from the 2009 polls over the rioting allegations. Last month, another accused, Yashpal Singh was sentenced to death for murder and rioting.
Sikhs make up some two percent of Hindu-majority India's population of 1.25 billion.