NOTE: This article is about the incarnation of the Mandarin from the Marvel Cinematic film series. The mainstream version can be found here: Mandarin (Marvel).
So, you mean to tell me that you don't know the history of the Mandarin himself? He was a warrior-king..... inspired generations of men through the Middle Ages, perhaps even further back in time.
~ Jackson Norriss explaining about the Mandarin.
The Mandarin is a major antagonist in the Marvel Cinematic Universe, mostly serving as the unseen overarching antagonist of the Iron Man trilogy and is set to appear in the upcoming 2021 film Shang-Chi and the Legend of the Ten Rings, presumably as the main antagonist.
He is the founder and leader of the terrorist organization Ten Rings, and is currently at large due to his identity remaining anonymous to the public.
He will be portrayed by Tony Leung Chiu-wai who will make his Hollywood debut in this role.
According to Jackson Norriss, there was a rumor that the Mandarin had many followers, who described him as an ancient warrior-king that has inspired generations of men since the Middle Ages, perhaps even further back in time. It is unknown if that rumor was true or not.
The Mandarin doesn't appear, nor is mentioned in Iron Man, but he sets his plot in furthering the goals of the Ten Rings by arranging Raza and a couple of terrorists to kidnap Tony Stark and hold him for ransom in an attempt to gain advanced weapons for their own goals (thanks to a deal with Obadiah Stane). However, Stark escaped captivity by using his new advanced armor to defeat Stane and the terrorists.
Iron Man 2
The Mandarin doesn't appear, nor is mentioned in Iron Man 2, but it is implied that he sent one of his agents to aid Ivan Vanko to kill Stark at the Prix de Monaco. To that end, the agent gave Vanko a pass to enter the race for the opportunity. However, Stark foiled this with his armor again before using his new armor (along with Rhodes') to defeat Vanko.
Iron Man 3
The Mandarin is mentioned in Iron Man 3, where Aldrich Killian hired a drunken actor named Trevor Slattery to impersonate the Mandarin as part of Killian's true plan of using Extremis experiments as purposeful attacks on the U.S. government. Needless to say, Stark managed to avoid this by purposely self-destructing all of his armor to destroy the Extremis soldiers, allowing Pepper to defeat Killian for good.
All Hail the King
The Mandarin serves as the unseen overarching antagonist of the short film, as he is very upset at Slattery (who is currently in Seagate Prison) for (unintentionally) stealing his name and making a mockery of his beliefs. As such, he sends Jackson Norriss to break Slattery out of prison and bring him over in person.
Due to not being identified or having fully appeared in any MCU installment, the Mandarin could possibly have a larger role in the franchise's future after Avengers: Endgame. Considering that Iron Man, his archenemy in the comics, died in such film, however, it's unlikely that he will ever appear unless he becomes enemy of another main character.
Kevin Feige confirmed on his Reddit account in 2019 that Marvel Studios has plans to feature the real Mandarin in the future.
Initially, Aldrich Killian was intended to be the franchise's version of the Mandarin. However, due to backlash from fans, Marvel changed this by releasing the one-shot All Hail the King featuring Ben Kingsley's return as Trevor Slattery, in which the short film revealed that the real Mandarin is a separate individual.