From Halopedia, the Halo wiki
Chef Pretty Officer Mister Chief Spartan 117 (pronounced "one-seventeen"), better known as Mister Chief, is a character created by the then-Bungie employee Frank O'Connor that recurringly appeared in Bungie Weekly Updates.
On 1/16/04, Frank O'Connor posted his first weekly update. At the time, Halo 2 was still under a veil of secrecy. Frankie had just seen a promotional poster by Lorraine McLees and wanted to include it his weekly update, however it was not possible at the time. So instead Frankie created a horrible artist's impression of it. Instead of Master Chief holding a gun, Frankie drew him with a lightsaber from Star Wars, "because guns are hard to draw". It did not look quite right so Frankie drew in some eyes and mouth and labeled it "Mister Chief".
O'Connor got such a response from fans that he decided to include another Mister Chief image in his weekly updates from then on. Mister Chief has been made in many different styles, including imitation Caravaggio and Japanese manga. Most ideas come from fan requests.
Mister Chief has been replaced in a couple weekly updates by Mrs. Chief, Evil Chief, and HotTub Guy (something which he believed San'Shyuum to resemble).
During PAX Prime 2014, a member of the 405th.com Halo fan costuming community wore a Mister Chief costume when proposing to his girlfriend on the stage of 343 Industries' Halo panel. Frank O'Connor, who headed the panel, commented that "No polygons were harmed during the creation of that costume". Two Mister Chief avatars were later included in an update to Halo: The Master Chief Collection. Season 5, released in 2021, will include a Mister Chief face as a helmet attachment for the Mariner helmet.
The Warzone Firefight update released on June 29, 2016 includes a Mister Chief easter egg in the Score Attack game mode; when the player(s) beats any mission of the campaign on LASO level (Legendary, All Skulls On), the post-carnage report background shows Mister Chief on a sheriff suit with a cowboy lasso.