He attended the University of Glasgow in Scotland and earned his degree in divinity in 1749. He was then ordained a Presbyterian minister at Ballykelly, County Londonderry, in 1752.
About 1750, he married Shirley Stirling who was the daughter of a Presbyterian minister from Walworth, Ballykelly. They had one daughter Mary (nicknamed Polly) who was born about 1752. It is believed that his wife Shirley passed away during childbirth. "Polly" was then raised by her uncle Samuel and his family.
In the 1757 he came to America and evidently resided somewhere in Pennsylvania as he served as a Captain in the Pennsylvania militia during the French and Indian war. He was part of the Forbes expedition that captured Fort Duquesne in 1758.
By 1764, he had settled near Milford, Delaware, and married Jemima Molleston, the widow of John Brinkle and sister of Henry Molleston. They had four children together.
In 1767, he bought a tract of land called "Longfield," now inside the northern limits of Milford just off Roosa Road.
When the Delaware General Assembly raised the 1st Delaware Regiment in response to the request by the Continental Congress they placed Haslet at its command on January 19, 1776, with the rank of Colonel.
The 1st Delaware Regiment under his command participated in the following battles: Battle of Long Island, Battle at Mamaroneck and White Plains, New York, Attack of Trenton, New Jersey (After crossing the Delaware with General George Washington), and the Battle of Princeton, New Jersey.
It was during the Battle of Princeton that Colonel John Haslet was fatally wounded and died.
Haslet was first buried at the First Presbyterian Church cemetery in Philadelphia, Pa. By an act of the Delaware General Assembly on July 1, 1841, his remains were disinterred and moved to the Presbyterian Cemetery here in Dover, Delaware.