In about 1230 Gilchrist Maclachlan was witness to a charter of Kilfinan granted by Laumanus, ancestor of Clan Lamont. The first documentary evidence of the clan's ownership of lands was recorded in 1292, when Gilleskel Maclauchlan received a charter of his lands in Ergadia from John, King of Scots.
According to the historian G. W. S. Barrow, Gillespie Maclachlan appears in the Ragman Rolls, when the magnates of Scotland signed their allegiance to Edward I of England, in 1296, "clerks of this period writing Anglo-French documents often had difficulty with the name Lachlan, and rendered it by some form of the more familiar name Rothland, or Roland. Thus, unnoticed by historians of Clan Lachlan, Gillespie MacLachlan figures on the Ragman Roll as 'Gilascope fiz Rouland, de counte de Perth'".
Sometime between 1306 and 1322 Gillespie received, in charter from Robert I of Scotland, the ten pennyland of "Schyrwaghthyne" (Strathlachlan) and other lands. He also appears on the list of Scottish magnates who sat at the first Parliament of the king of Scots at St Andrews, in 1309. Gillespie was one of the sixteen Scottish magnates who signed a letter to Philip IV of France in 1309. The King of France had asked for Scottish assistance in a Crusade he was forming, with the Scots answering that they were at war with England and had their hands full. His name appears on one of the seal tags with that letter, though the actual seal that had been attached to the tag has since been lost.
In 1314 "Guyllascop Maclouchlan in Ergadia" (Gillespie Maclachlan of Argyll) granted forty shillings sterling to the Preaching Friars of Glasgow, the sum of which were to be paid from his pennylands of Killbride near Castle Lachlan. ("juxta castrum meum quod dicitur Castellachlan"). Gillespie was dead by 1322 and was succeeded by Patrick his brother. Patrick married a daughter of James the Steward of Scotland, and had a son, Lachlan, who later succeeded him. In 1410 Iain Maclachlan, lord of Strathlachlan, ("Johonne Lachlani domino de Straithlaon"), witnessed a Lamont charter. In 1456 Lachlan's son, "Donaldus Maclachlane dominus de Ardlawan" ("Ardlachlan", or Castle Lachlan), like his ancestor Gillespie, granted the Preaching Friars of Glasgow six shillings and eight pence per year, from the same pennylands of Killbride beside his home Castle Lachlan.
One tradition of the Maclachlan lairds was thought to date from the era of the Crusades. The tradition was that the laird of Strathlachlan (Maclachlan of Strathlachlan) and the laird of Strachur (Campbell of Strachur) would attend the funerals of each other and "lay his neighbour's head in the grave". This tradition was thought to originate from the Crusades because, "it is said the heads of these two families went together to the war, and each solemnly engaged with the other to lay him in his family burying-place if he should fall in battle".