Was the second son of Dafedar (cavalry sergeant) Hardit Singh and Mata Prem Kaur of Dharovali village in Gurdaspur district. He learnt to read the scripture in the village gurdwara. He was married on 26 February 1886 to Bibi Basant Kaur, daughter of Bhai Harnam Singh of Muhaddipur village in Jalandhar district. The family later migrated to Chakk No. 33 Dharovali in Sheikhupura district in the Lower Chenab Canal Colony.

Ishar Singh was of deeply religious temperament, and although he had taken the Khalsa pahul at the hands of Bhai Mul Singh Garmula, he was more impressed by his grandfather, Naurang Singh, who renouncing his home had joined a band of sadhus and gone towards Haridvar. Ishar Singh too after some time went to the Kumbh fair at Paryag (Allahabad) in search of his grandfather. For three years he roamed about the country with holy men, and ultimately reached Sri Abchalnagar Hazur Sahib, Nanded, in the then Hyderabad state.

His uncle, Kesar Singh, who was serving in 30th Cavalry, then stationed at Nanded, recognized him and reclaimed him to the Sikh fold. Ishar Singh took the pahul again at Hazur Sahib and returned home accompanying his uncle when the latter was granted leave some six months later. He now settled down as a householder, but his religious zeal was undiminished. He joined hands with Bhai Lachhman Singh (who was a collateral nephew to him) in organizing the historic conference held at their village on 1 to 3 October 1920 for propagating the cause of Gurdwara Reform. Together they mobilized support and enrolled volunteers for the liberation of Gurdwara Janam Asthan, Nankana Sahib.

On 19 February 1921, while Lachhman Singh collected and led volunteers from villages in the immediate neighbourhood of Dharovali, Bhai Ishar Singh collected another 30 odd men from villages further to the west. These latter formed a separate jatha and chose Ishar Singh as their jathedar (leader). During the night 19-20 February, this jatha, although supposed to join the other led by Bhai Lachhman Singh, lost their way and were yet about one kilometre short of Janam Asthan when the other one had already been shut in and was being massacred by the Mahant's hirelings. Hearing the bangs of gunfire, these men ran forward to join their comrades. Jathedar Ishar Singh being the oldest among them lagged behind so that when he reached near the Gurdwara, he found the younger lot coming back on the run chased by the assassins. They told him that the situation was hopeless and they should go back, but he continued to run forward shouting that they had come to die and die they must. He faced the pursuers and bore a bullet from one of them on his chest before he fell down; the others hacked him to pieces and dragged his body to a burning pyre.

The Shiromani Gurdwara Parbandhak Committee granted a pension to Bhai Ishar Singh's widow at Rs 150 per annum.

Source: Encyclopaedia of Sikhism, Harbans Singh