Monks first Appeared in the Blackmoor Supplement in OD&D (1975)
Monks (Order of Monastic Martial Arts), a sub-class of Clerics which also combines the general attributes of Thief and Fighting Man (Blackmoor, p. 1)
Monks were introduced as a sub-class of cleric. They had a number of unique skills/powers that have carried forward through the editions, to include stunning opponents, unarmed combat, having some thief (rogue) class skills, and an armor class that increased with level or ability score while the character was basically unarmored. We had a monk in our high school group who was effective (our PC party had 6 or 7 depending on who showed up) but also very fiddly: dodging missiles and spells was a special defense only the Monk got; even a failed save gave a monk only half damage from a fireball. His surprise rules (we rolled then) were different; he added fractions of damage to hits based on his level - a level 5 monk did 2.5 damage bonus to any hit; his stun ability was very much hit or miss; he could kill an opponent with a single blow!
... any score 25% (5 or better) above the minimum required for a hit has a 75% chance of stunning the opponent for from 3-12 turns and a 25% chance of killing the opponent.
2. damage otherwise done is determined on a special table... (Blackmoor, p. 1)
Monks could not be psionics when Eldritch Wizardry came out. (EW, p. 2)
With d4 hit dice per level, the Monk was a glass cannon and a scout.
- About those d4 hit dice: at that point in time (Greyhawk was out)
Monks, Thieves, and Magic Users had d4 hit dice; Assassins, Clerics
and Druids had d6 HD; and Fighting Men (plus Paladins) had d8 hit
It was hard to qualify for this class: a minimum of 15 Wisdom, 12 Strength, and 15 Dexterity was called for. When rolling 3d6 in order, that wasn't a common occurrence. The class was restricted to humans. The Multiple Ability Score Dependency problem that it ran into in later editions was a "feature/bug" from the beginning.
The Monk grew in complexity somewhat, however it was very recognizable as the OD&D monk. The race restriction to Human remained in place. The monk started with 2d4 HD, and then went up one d4 per level.
The Quivering Palm ability at high level was still quite lethal; the trick was to get a monk to survive to that high of a level. Having to fight other NPC monks to go up in level made the monk unattractive to some parties/tables. (I probably played most of my D&D in this edition, if you count both player and DM, and rarely saw anyone select this class). Having to meet class minimum scores reduced how often one even considered a monk (similar issue applied to Paladins, Rangers, and Illusionists). Minimums were : 15 STR, DEX, and WIS and 11 CON. The AD&D 1e DMG allowed for the roll 4d6 drop 1 approach (and I saw it used frequently) so the chances were a little better that one could qualify for this class. The restriction on Monks having psionic powers was not in the PHB Appendix for that optional rule. If one was using the grappling rule from the DMG...
... Even if grappled, pummeled, or overborne, monks are able to conduct open hand combat normally until stunned or unconscious. (p. 72)
The DMG also had a rule that a Monk could only "kill with one blow/stun" a creature of human size, and then a bit larger as levels went up. By level 10, the monk wasn't knocking off Storm Giants any longer in this edition, if the DM applied that rule.
Like the Assassin, the Monk was not in the Core rules (PHB). It did return as "Fighting Monk" in the Complete Priests Handbook. (I never saw one in play).
The Minimum ability score of 12 Dexterity was hardly a barrier to entry. No racial barriers for PCs. Bankuei did a nice job of explaining the "choice" to play a monk in this edition here, pointing out that classes are not carefully balanced against each other in the earlier editions.
This was a Priest sub class; hit dice went from d4 to d6.
A point about differences
From this point forward (WoTC Monks) the "minimum scores to qualify for this class" principle was discarded. This means that you could choose the Monk and work with that class and its features as with any other class, whether you rolled or point bought or used a standard array for character ability scores. (Likewise with Ranger and Paladin). From this point forward, hit dice are d8.
While getting out from under the Cleric/Priest sub class was a step forward, the Monk took a step back in combat effectiveness during this edition. No few posts here (and at various D&D forums) describe the shortcomings of the class. (For example, the flurry of blows and fast movement preempt each other). The Monk was a poor choice for a martial character in this edition if one was going by the Tier system of PC class ranking. Multiple Ability Score dependency was one of a number of problems the class ran into. Pathfinder's Unchained Monk addressed some of those weaknesses, putting the Monk into Tier 3 for that variation of the D&D 3.X system.
While balance in this edition was less of an issue, the Monk used Strength for basic melee attacks/opportunity attack; basic at-will powers were Dexterity based attacks/damage. Its role was Striker. Its play could be described as "get in to dance between swords and land decisive strikes with mystical martial arts" (Thanks @Powerdork). The Monk was a psionic class in this edition. (Thanks @KRyan).
The Monk had a unique mechanic: “full discipline” powers. This gave two actions that had to be used together (usually a standard and move action, but not always) which emphasized mobility and multiple attacks. The Flurry of Blows power added damage after a successful attack. (Thanks. @Guybrush McKenzie)
The archetype of an unarmored skirmisher with decent armor class, an effective melee combatant although playing "tank" is risky. The ability to stun opponents - applying a condition that is powerful in this edition - is a huge benefit to the party in fighting against spell casters or a single boss monster1. The flurry of blows and a variety of ki/magical abilities and powers, and spell like abilities, give the Monk a hint of a psionic feel.
A significant improvement "out of the box" was the Martial Arts feature at level 1 that allowed dexterity to be applied to melee attacks and damage, and a built in bonus attack unarmed strike.
The recharge of ki points on a short rest makes this class, like the Warlock and the Fighter, far more effective in a game where 1 or 2 short rests happen during an adventure day (per the general design) rather than the 15 Minute Work day style of campaigns. (The last monk I played for an extended campaign suffered from a lack of short rests based on how the DM ran that adventure). In this edition, the Monk can be played from level 1 through 20 more or less independent of items.
1 On an experiential note, the stun effect remains nearly as powerful as the original OD&D Monk (absent the "it might die!" feature) in terms of how it lets the party go "nova" on a single target. We took out a mini-BBEG in a one shot thanks to my monk going "all in" to apply ki to attempts to Stun. The nova from the other characters did the rest once "stun" was in place.