The name Leslye originated from the Ancient Brittonic language, spoken in Britain before the Anglo-Saxon period. It is derived from the Brittonic word "lès", meaning "courtyard" or "prince's court". The name likely emerged during the early medieval period in the 5th or 6th century AD.

In its earliest form, the name was spelled as "Lēslic" or "Lēslicc" in Old English. Over time, it evolved into the more modern spelling of "Leslye". The name gained popularity among the Norman nobility after the Norman Conquest of England in 1066, as the Normans adopted and adapted many Brittonic names.

One of the earliest recorded instances of the name Leslye can be found in the Domesday Book, a manuscript record of the Great Survey of England commissioned by William the Conqueror in 1086. The name appears as "Leslic" in this historical document.

Throughout history, several notable individuals have borne the name Leslye. One of the earliest was Leslye de Avranches (c. 1050 - c. 1120), a Norman nobleman who participated in the Norman Conquest of England and became the first Lord of Folkestone.

Another famous bearer of the name was Sir Leslye de Mohun (c. 1240 - 1295), an English knight and landowner who played a role in the Second Barons' War against King Henry III.

During the Scottish Wars of Independence, Sir Leslye de Penicuik (c. 1280 - 1347) was a Scottish knight who fought alongside Robert the Bruce at the Battle of Bannockburn in 1314.

In the 15th century, Leslye Stewart (c. 1440 - 1497) was a Scottish noblewoman and the Countess of Buchan. She was known for her involvement in the coronation of Robert the Bruce.

Leslye Groves (1896 - 1970) was a British military engineer who played a crucial role in the Manhattan Project, overseeing the construction of the facilities used to produce the first atomic bombs during World War II.