"He was at a marriage function when he fell ill and was rushed to a hospital in Maubhandar but he died on the way", his private secretary Kablu Mahato said.
The 58-year-old JMM leader is survived by wife and four daughters.
Mahato first won assembly polls in 1990 from Ichchagarh and retained the seat in 2005 and became the Deputy Chief Minister.
He lost the seat in the 2009 polls.
JMM's spokesman Vinod Pandey condoled his death.