Chainmail is a medieval miniature wargame created by Gary Gygax and Jeff Perren. Gygax developed the core medieval system of the game by expanding on rules authored by his fellow Lake Geneva Tactical Studies Association (LGTSA) member Jeff Perren, a hobby-shop owner with whom he had become friendly. Guidon Games released the first edition of Chainmail in 1971.

In 1967, Henry Bodenstedt created the medieval wargame Siege of Bodenburg, which was designed for use with 40mm miniatures. Gary Gygax first encountered Siege of Bodenburg at Gen Con I (1968), and played the game during that convention. The rules for Siege of Bodenburg had been published in Strategy & Tactics magazine, and Jeff Perren developed his own medieval rules based on those and shared them with Gary Gygax.

The original set of medieval miniatures rules by Jeff Perren were just four pages. Gygax edited and expanded these rules, which were published as "Geneva Medieval Miniatures", in Panzerfaust magazine (April 1970), using 1:20 figure scale. The rules were again revised, and then self-published in the newsletter of the Castle & Crusade Society, The Domesday Book, as the "LGTSA Miniatures Rules", in issue #5 (July 1970), using 1:10 figure scale. Later issues of The Domesday Book introduce a rule system for man-to-man combat at 1:1 figure scale and a rule system for jousting.

Gary Gygax met Don Lowry at Gen Con III (1970), and Gygax later signed with Lowry when he founded Guidon Games to produce a series of rules called "Wargaming with Miniatures". The first game published was a further expansion of the medieval rules, published as Chainmail. Guidon Games released the first edition of Chainmail in 1971 as its first miniature wargame and one of its three debut products.

Along with the previous medieval rules, Chainmail included a 14-page "fantasy supplement" including figures such as heroes, superheroes, and wizards. The fantasy supplement also included mythical creatures such as elves, orcs, and dragons. The fantasy supplement also referenced the works of J. R. R. Tolkien, Robert E. Howard, Poul Anderson, and Michael Moorcock. The fantasy supplement encouraged players to refight fixed battles based on fantasy fiction by J. R. R. Tolkien, Robert E. Howard, and other writers.

The Chainmail cover art of a fighting crusader was inspired by a Jack Coggins illustration from his book The Fighting Man: An Illustrated History of the World's Greatest Fighting Forces. Both Perrin and Gygax "swiped" Coggin's artwork to illustrate their preliminary articles about Chainmail that appeared in Panzerfaust and The Domesday Book. When Don Lowry of Guidon Games agreed to publish Chainmail, Lowry swiped the same Coggins illustration for the cover. For the fantasy supplement, the illustration of a mounted knight charging towards a dragon, was drawn by Don Lowry, based heavily on an illustration by Pauline Baynes for J. R. R. Tolkien's Farmer Giles of Ham (1949).

First edition Chainmail saw print in March 1971. It quickly became Guidon Games' biggest hit, selling one hundred copies per month.

Guidon Games published Chainmail second edition in 1972.