Summary
It seems like these days everyopen-world gamehas a multiplayer component of some sort, including cooperative and competitive modes. Some are even built around it. This was not always the case, however. After all, it is no simple feat to let two or more people run around a large environment.
The games below managed to accomplish open-world multiplayer before it was the norm. Some of these games might not seem that old, but it just shows how novel the ubiquity of multiplayer in open-world games is, whether that be locally on the same screen and console or online across the world.

Though all the games below have multiplayer, not all of the modes are specifically about letting two players use the entire map at once. Additionally, these are all open-world games and not games with large hubs as levels, so do not expect to seeDestroy All Humans 2.
Grand Theft Auto 5’smultiplayer was revolutionary, andGrand Theft Auto 4’smultiplayer offerings feel like the progenitor to that.San Andreason the PS2 had a multiplayer mode on the prior console generation that few people talk about.

Certain Rampage missions can be tackled by two players at once. This comes with its limitations, though. It is not split-screen, so it fundamentally changes the way people have to play. It ultimately feels like an afterthought, but it is nice to have nonetheless.
It is hard to describe just how uniqueBorderlandswas when it first came out. The idea of taking dungeon crawling game mechanics and putting theminto a cooperative shootermade for a brand new type of gameplay loop.

Add that into an open-world setting (albeit one split into separate ideas rather than one continuous map), and add multiplayer on top of it, and Gearboxcreated a formulathatnumerous games have borrowed from, likeDead Island, Dying Light,andOutriders. Split-screen options hit the frame rate hard in those days, but it was a worthy concession for the ability to play with a friend.
The original release on consoles is not ideal for co-op with the effect it has on performance, but re-releases on modern systems allow for four players to play split-screen without tanking the frame rate like it used to on PS3 and Xbox 360.

Open-world racing gameswere rare on the PS2 and Xbox, which is what makesNeed For Speed Underground 2such a special game. It laid the groundwork for future racing games that would adopt the format likeMidnight Club 3.
The multiplayer is not the same as modern open-world games, though. Instead, races and modes cut off pieces of the map to use as tracks. It is not an unreasonable trade-off for a game this old. These days, multiplayer modes in many open-world racing games do indeed allow for multiple racers to drive around the whole map unhindered by limitations.

This open-worldSimpsonsgameis a classic most gamers from the PS2 and Xbox generations fondly remember. It was a great alternative for any player who was too young to enjoy theGrand Theft Autoseries or any of its copycats likeTrue Crime: Streets of LA.
The multiplayer was only a small portion of the game and did not allow players to roam around the open-world Springfield. It was relegated purely to a kart racing mini-game players unlocked through collecting cards throughout the world. It is only a tiny portion of the game most players can miss, but at least it is there for anyone interested.

Tony Hawk’s American Wastelandwas the first open-world game in the series (though the levels inTony Hawk’s Underground 2were large enough to be small open-worlds). The series has always had great multiplayer modes, and the online modes added evenmore competitive offeringsthat were completely closed off to those who were only playing locally.
In addition to the open world where the story takes place, the game is also loaded with classic maps from prior games in the series. There is also a cooperative offering where the Classic Mode can be tackled by two players simultaneously.

BeforeGrand Theft Auto 4came out in 2008, players were able to satiate their open-world hunger on the Xbox 360 for a couple of years withSaints Row.Cooperative multiplayer was always a staple of the series and it was always simple to go through the story with a friend and to be in completely different areas of the map.
In the first game of the series, however, multiplayer was its own separate mode and not integrated into the storylike in its sequels.All the same, it established the series' dedication to letting friends play the game together.
Burnout Paradisetook the series in an open-world direction and fully implemented this design into the multiplayer. Players could join a lobby where they would just roam around the map with other players before the host initiated a race. To make the free-roam portion entertaining, a variety of challenges are presented on the map for players to accomplish.
Some of them even required teamwork from multiple drivers to accomplish. It was refreshing to see the multiplayer component included in its re-release, especially when a lot of remasters eschew online multiplayer. It is just a shame theBurnoutseries fizzled out after this entry.