Mr Singh, 63, and his two brothers were held guilty last week, after a two-decade trial in the murder of former Janata Dal lawmaker Ashok Singh in 1995. Ashok Singh's wife had asked for the trial to be held on neutral ground in Jharkhand, instead of Bihar.
Lalu Yadav, a partner in Bihar's ruling coalition, had said that there was no need for action against Prabhunath Singh as he was "likely to appeal in the High Court and Supreme Court."
Mr Singh, his brother Dinanath Singh, and another accused Ritesh Singh - were produced in court from jail through video link.
The court in Hazaribagh also sentenced all three to 10 years' rigorous imprisonment for keeping and using explosives. The three are to pay Rs 40,000 fine.
Ashok Singh had defeated Prabhunath Singh in the Bihar assembly poll in 1995. He was killed in a blast at his official home in Patna soon after.
Prabhunath Singh had allegedly said that Ashok Singh would be eliminated within 90 days of the election.
Ashok Singh's wife Chandni Devi filed a case with the police in Patna.
On the Supreme Court's directive, the case was transferred from Patna to Hazaribagh in 1997. The charges were framed in 2008.
The family members of those sentenced said they will move the High Court against the judgment.
Prabhunath Singh was in the Janata Dal in 1995 and later joined Janata Dal-United (JD-U) of Nitish Kumar before finally going to the RJD.