Singh, 68, was admitted to AIIMS on September 18, SP spokesman Rajendra Chaudhary. Singh is survived by his wife Urmila and two daughters.
AIIMS medical superintendent PK Sharma said,"Singh was suffering from blood cancer."
"Two days back his condition became serious and he was shifted to ICU. He died at 4:15 pm today."
His body will be taken to Lucknow on Monday and put in party office at Vikramaditya Marg, SP general secretary Ram Gopal Yadav said.
The 68-year-old leader was suffering from blood cancer
Later, the body will be taken to Gorakhpur and then to his ancestral village Jainagar in Deoria for the last rites.
"After the demise of Janeshwar Mishra and Brij Bhushan Tiwari, the demise of Mohan Singh is a setback for the socialist movement. He was a man of principles and his death has left a void in the Samajwadi Party," Ram Gopal said.
SP supremo Mulayam Singh Yadav, UP Chief Minister Akhilesh Yadav, UP Assembly Speaker Mata Prasad Pandey, LJP leader Ram Vilas Paswan and several party leaders have condoled his death.
On January 6, 2012 Singh was removed from the post of SP national spokesman after he disagreed with Akhilesh Yadav's decision to keep controversial leader DP Yadav out of the Samajwadi Party, who accused party leadership of dictatorship.
Singh entered politics when he was president of Allahabad University Students' Union during 1968-69 and was inspired by father of Indian socialist movement Raj Narain and Ram Manohar Lohia.
He went to jail during Emergency.
On earlier occasions too, Singh was jailed while participating in agitations, including the one in 1966 when he tried to occupy Anand Bhavan, Allahabad.
Singh, a veteran socialist leader and a friend of Madhu Limye, was a three-time Lok Sabha MP from Deoria. He also served as minister in UP cabinet.
He was a member of UP Legislative Assembly in the late Seventies. Singh had been associated with Lok Dal, Janata Party and Samata Party.
He was awarded the Best Parliamentarian award in 2008.
A keen reader and orator, Singh has authored several books including 'Bharatiya Samvidhan ke Nirman Mein Nehru', 'Bhartiya Loktantra Ka Sankat; 'History of Socialist Movement', 'Dr Ambedkar-A multifaceted personality' and 'Freedom Movement and Socialist Contribution'.
BJP's prime ministerial candidate Narendra Modi expressed condolences to Singh's family.