Dhillon was from the village of district Taran Tarn (erstwhile Amritsar). He was born in 1883 in the home of Sardar Aroor Singh Dhillon and Sardarni Gulab Kaur. At the age of twenty, he joined 35 British Cavalry. After resigning, he migrated to Shanghai and then further to Vancouver. At Westminster city, he worked in a lumber mill. Here, with the help of community members, he established a Gurdwara. He was one of the seven members of the Gurdwara Committee to look after its day-to-day affairs. Canada’s democratic environment but the hypocritic character of the government caused much resentment in his mind.
Newly acquired political consciousness in Canada pushed him towards the Ghadar Party which was established in America by Sohan Singh Bhakna. Jagat Singh became an active member of the organization. With the outbreak of World War I, Ghadar Party asked its cadre to reach India to foment a revolution. Before boarding to India, Jagat Singh, on 17 August 1914, killed Harnam Singh, who was working as an agent for British intelligence officer William Hopkinson.
On 1 September 1914, he reached India. Here, he joined Kartar Singh Sarabha and other party members. Along with his two comrades, he visited Ferozepur railway station to collect a packet of weapons. But here, policemen were patrolling with the station master. Ghadarites, in suspicion, killed the station master and fled. Later, he took part in nationalistic dacoities at Sahnewal in Ludhiana and Chabba in Amritsar. He distributed Ghadar literature in villages and appealed to the people to rise against British rule. In a meeting, he offered to mortgage his land to raise money for the publication of a paper in Punjab. Ghadarites decided to attack Mian Meer’s cantonment on 26 November 1914. For the execution of this plan, Jagat Singh along with some other revolutionaries reached there but this plan was shelved due to certain reservations of the army personnel.
When Michael O’Dwyer got the whiff of the 21 February 1915 rebellion by the ghadarites, he disarmed the soldiers of all cantonments. Many Ghadar Party members were rounded off. Jagat Singh, along with Kartar Singh Sarabha and Harnam Singh ‘Tundilat’ went towards Afghanistan. But the trio resolved not to flee and to lay down their lives for the liberation of their country. They reached Chak no. 5 of Sargodha district to meet Jagat Singh’s old friend to procure rifles. Here, Rasaldar Ganda Singh informed the police. Jagat Singh, along with Kartar Sigh Sarabha and Harnam Singh ‘Tundilat’ were arrested and tried in the First Lahore Conspiracy Case. He was hanged along with the other six ghadarites on 16 November 1915 in Lahore Central Jail.