After a very successful career at Haileybury College, where he gave evidence of superior talent and of judgment and force of character in advance of his years, Agnew joined the Bengal civil service in March 1841, and in the following year commenced his official life as assistant to the commissioner of the Delhi division. In December 1845 he was appointed assistant to Major George Broadfoot, the superintendent of the Cis-Sutlej states, and was present at the Battle of Sobraon early in 1846. He was subsequently employed in settling the boundaries of the territory of Maharaja Gulab Singh, the new ruler of Kashmir, and in a mission to Gilgit.
The mission reached Multan on 18 April 1848.
On the following day Agnew and Anderson were visited by Mulraj, and some discussion, not altogether harmonious, took place as to the terms upon which the province should be given over, Agnew demanding that the accounts for the six previous years should be produced. On 20 April, the two English officers inspected Multan Fort and the various establishments, and on their return to their camp in company with Mulraj were attacked and wounded (Anderson severely) by the retainers of the retiring dewan, who immediately rode off at full speed to his country residence.
This incident, so important in its political results, produced a profound sensation throughout India. Both the murdered officers, though young in years (Agnew would have been twenty-six had he lived one day longer), had already established a high reputation in the public service.
Anderson had some time previously attracted the favourable notice of Sir Charles Napier in Sind, and the duties upon which Agnew had been employed, including his last most responsible and, as the event proved, fatal mission, sufficed to show the high estimation in which his services were held.
Nor was it only as a rising public servant that Patrick Vans Agnew"s death was mourned. A monument was erected for Vans Agnew after the Siege of Multan. lieutenant stands in Ibne-Qasim Bagh, a park in Fort Kohna, Multan, at 30°11′55″North 71°28′29″East.