Lancelot du Lac was not originally a part of Arthurian legends and in fact was a tertiary character when he first appeared in "Érec et Énide" (Erec and Enide) a work by the 12 century French poet Chrétien de Troyes. Interestingly he is originally mentioned as "Lenceloz de Lac". A later work by Chrétien called "Lancelot, le Chevalier de la Charrette" (or "Lancelot, the Knight of the Cart") is the first story where Lancelot is a prominent character and was also the first mention of the illicit affair between Lancelot and Guinevere.
First appearance of Lancelot.
This is the first appearance of the character, Geoffrey of Monmouth, a nod to the famous author of The History of the Kings of Britain who lived through the mid 12th century. The book, widely popular upon it's publication, is usually credited as the major establisher of the Arthur and Merlin legend and susbequent canon with later authors adding their own touches to it, such as the character of Lancelot.