One of the medievalists might be able to chime in on how leather armor is misrepresented in popular culture, but it isn't true leather was never used as armor. During the 17th century it was popular for soldiers to wear a "buff coat" which was made of thick, soft leather and provided moderate protection against sword cuts. In the late 16th century, William Garrard mentions ". . .the light armed pikes, who amongst some nations for want of brest plates of Iron, vse tand lether, paper, platecoates, iackets, [etc.]. For a gorget, thicke folded kerchefes a∣bout their neck, a scull of Iron for a head péece, and a Uenetian or lether Shéeld and Target at their backes, to vse with their short Swordes at the close of a battaile, and in a throng."

During the middle ages a layer of leather was often incorporated into the construction of shields or cloth armor for added protection. There are also references to "Cuir bouilli" which were pieces of hardened leather incorporated into armor, although I'll have to let someone else explain whether these were meant for combat or were primarily decorative.

David Jones in his 2012 article "Arrows against Linen and Leather Armour" included a references to leather being incorporated into armor. His own testing seemed to conclude that armor comprised of both linen and leather tended to protect better against most arrows than linen or leather alone.

Historical descriptions of linen and leather armour

The Táin Bó Cuailgne contains a description of linen and leather composite armour worn by the mythical Irish hero Cú Chulainn. O’Rahilly’s translation of the 10th or 11th century version is given by Halpin as:

‘Twenty-seven tunics [cneslenti] worn next to his skin, waxed, board-like, compact, which were bound with strings and ropes and thongs close to his fair skin… Over that outside he put on his hero’s battle-girdle [cathchriss] of hard leather, tough and tanned, made from the best part of seven ox-hides of yearlings, which covered him from the thin part of his side to the thick part of his arm-pit; he used to wear it to repel spears [gai], and points [rend] and darts [iaernn] and lances [sleg] and arrows [saiget], for they glanced from it as if they had struck against stone or rock or horn.’

If “the best part” means “the thickest part” this would be butt leather, the hide of the upper hindquarters of the ox, that is used for the soles of boots and shoes. Cú Chulainn was a son of the god Lug, and perhaps could bear a greater weight of oxhide than ordinary mortals.

Another description of linen and leather armour was given by the Scottish historian John Major in 1512. It was quoted and translated by Skene as:

‘In panno lineo multipliter intersuto et cocreato aut picato, cum cervinae pellis cooperatura vulgus sylvestrium Scotorum corpus tectum habens in praelium proslit.

The common people of the highland Scots rush into battle having their bodies clothed with a linen garment, manifoldly sewed and painted or daubed with pitch, with a covering of deer-skin.’

The most detailed instructions concerning the making of jacks are in a decree of Charles V of France (reg.1461-1483). It contains the following specification:

`Et premièrement leur faut des dits jacques trente toiles, ou de vingt-cinq, à un cuir de cerf a tout le moins: et si sont de trente-un cuirs de cerf ils sont de bons. Les toiles usees et déliées moyennement sont les meilleures; et doivent estre les jaques a quatre quartiers, et faut que manches soient fortes comme le corps, réservé le cuir.

And firstly the said jacks should be of thirty layers of linen, or twenty-five, to one deerskin at the least: and if they are of thirty-one deerskins they will be good. Worn and moderately unravelled linen is the best; and the jacks must be in four quarters, and the sleeves must be as strong as the body, except for the leather.`

The French “cerf” means specifically the red deer (Cervus elephas), and not the smaller fallow or roe deer. The requirement for 25 or 30 layers of linen was incorrectly given as 29 or 30 layers by ffoulkes, and this mistake has been perpetuated by other authors. The option of 31 layers of deerskin was regarded by ffoulkes as a probable clerical error.

Curzon was of the opinion that mail armour was often worn over a leather cuirass, and in 1869 he wrote:

‘A solitary fragment of one of these is in the possession of Mr. Eastwood, which was found in a stone coffin with the chain armour of a knight of the thirteenth century; it is made of two or three thicknesses of leather like that used for the soles of shoes, sewed together with leather thongs.’

The location and archaeological details of the find are lacking.

https://www.academia.edu/5520314/Arrows_Against_Linen_and_Leather_Armour