Mechanized Infantry Units, also known as Goliaths and Walkers or shortened to Mechs and MIUs, are the most prominent feature of the Steel Goliaths series. Mechs have the ability to use a wide range of energy, ballistic, and explosive weaponry, as well as different types of armor, shields, engines, and cores. These features, along with a Mech's overall tonnage, puts the Mech into one of five categories: Light Mechs, Medium Mechs, Heavy Mechs, Assault Mechs, and Support Mechs. In Steel Goliaths II, a sixth category was added known as Destroyer Mechs, but this category did not appear in the third game. Each category and mech has a different role on the battlefield and how one is used can quickly change the terms of engagement. However, a Mech's role does not define how a pilot uses their Mech and, as such, Mechs of different categories can be found doing each others' roles.
At the time of the first game's release, a total of 88 Mechs were planned to be featured in the game, but only 52 made the final cut.