Sahibzada Ajit Singh was born to Mata Sundari and Guru Gobind Singh ji at Paonta Sahib on 26 January 1687. The following year, Guru Gobind Singh returned with the family to Anandpur where Ajit Singh was brought up in the approved Sikh style. He was taught religious texts, philosophy and history, and had training in the manly arts such as riding, swordsmanship and archery. He grew up into a handsome young man, strong intelligent and a natural leader of men.
Soon after the creation of Khalsa on 30 March 1699, he had his first test of skill. A Sikh Congregation Sangat coming from pothohar, northwest Punjab, was attacked and looted on the way by Ranghars of Nuh, a short distance from Anandpur across the River Sutlej. Guru Gobind singh ji sent Sahibzada Ajit Singh, barely 12 years old, to that village. Ajit Singh along with 100 Sikhs reached there on 23 May 1699, punished the Ranghars and recovered the looted property.
A harder task was entrusted to him the following year when the hill chiefs supported by imperial Mughal troops attacked Anandpur. Sahibzada Ajit Singh was made responsible for the defence of Taragarh Castle which became the first target of attack. This according to the Bhai Vahis, happened on 29 August 1700. Ajit Singh assisted by Bhai Ude Singh, a seasoned soldier, repulsed the attack. He also fought valiantly in the battle of Nirmohgarh in October 1700. On 15 March 1701, a Sikh Sangat, column of Sikh devotees, coming from Darap area (around Sialkote) was waylaid by Gujjars and Ranghars. Sahibzada Ajit Singh led a sucessful expedition against these thugs.
As instructed by his father Guru Gobind Singh on 7 March 1703, he led 100 horsemen to Bassi, near Hoshiarpur and rescued a young Brahman bride forcibly taken away by the local Pathan chieftain. In 1691, Mata Sundari gave birth to another boy which was named Jhujhar Singh. Sahibzada Jhujhar Singh was four years younger then Ajit Singh. Sahibzada Ajit Singh for him was a sort of role model to be followed. Jhujhar Singh also was entrusted with several engagements around Anandpur and on hills. Both Ajit Singh and Jhujhar Singh led hundreds of successful expeditions against tyrants, helping needy who would come to Guru Gobind Singh asking to get them justice.
Then Mughal forces sieged Anandpur in 1705, Sahibzada Ajit Singh displayed his qualities of courage and steadfastness. When, at last, after Mughal Faujdar give assurances that he only wanted the castle of Anandpur, Anandpur was vacated on the night of 3-4 December 1705. Sahibzada Ajit Singh was given the command of the rearguard. As the besiegers, violating their solemn promises for a safe conduct of the evacuees, attacked the column, he stoutly engaged them on a hill-feature called Shahi Tibbi until relieved by Bhai Ude Singh. Ajit Singh crossed the Sarsa, then in spate, along with his father, his younger brother, Jhujhar Singh, and some fifty Sikhs. Further reduced in numbers by casualties at the hands of pursuing troop from Ropar, the column reached Chamkaur in the evening of 6 December 1705, and took up position in a garhi, a high walled fortified house. The host, since swelled by reinforcements from Malerkotla and Sirhind and from among the local Ranghars and Gujjars, soon caught up with them and threw a tight ring around Chamkaur. An unequal but grim battle commenced with the sunrise on 7 December 1705-in the words of Guru Gobind Singh's Zafarnamah, a mere forty defying thousands. The besieged, after they had exhausted the meagre stock of ammunitions and arrows, made sallies in batches of five each to engage the encircling host with sword and spear.
Sahibzada Ajit Singh led one of the sallies and laid down his life fighting in the thick of the battle. Gurdwara Qatalgarh now marks the spot where he fell, followed by Sahibzada Jhujhar Singh who led the next sally. An annual fair is held in the commemoration of their martyrdoms on the 8th of the Bikrami month of Poh (December - January).
The martyrdom of two of the sons of Guru Gobind singh in the battle of Chamkaur is substantiated by a contemporary record in the form of an official letter preserved in a MS., Ahkam-i-Alamgiri by Emperor Aurungzeb's official letter writer, Mirza 'Inayat' Ullah Khan Ismi (1653-1725). The relevant extract from the Ms., translated into English, shows that Mughals supplied at least 700 men while Sarhind, Malerkotla, and Ranghars were rest, it reads:
Received the letter containing misclaneous matters including the arrival of Gobind, the worshipper of Nanak, to a place 12 kos from Sirhind; the dispatch of a force of 700 with artillery and other material; his being besieged and vanquished in the haveli [a large walled house] of a zamindar of village chamkaur and killing of his two sons and other companions; and the capture of his mother and another son...