The Engineers were created by the Forerunners prior to the first activation of the Halo Array. Being mechanical structures rather than true life forms, they survived the activation and were later found by the Prophets in various M-Series facilities.
In the Halo Fiction
Engineers were first introduced to Halo
fans in the Fall of Reach novel, the prequel to Halo: Combat Evolved
. On a mission, the Master Chief
recorded footage of an Engineer taking apart a human car and putting it back together. This was the first time the UNSC
had seen an Engineer.
In a later novel, Contact: Harvest, it was revealed that Engineers were responsible for making the first ever Brute Choppers. An Engineer, Lighter than Some, had tried to reassemble and improve on the farming equipment for the people of Contact: Harvest as a peace offering. Unfortunately for the humans, the captain of the Brute warship discovered the vehicles and claimed them for his use.
In the Halo Games
Engineers were never seen in a Halo game until Ensemble's Halo Wars
. Prior to Halo Wars
, they had been repeatedly cut from Bungie's Halo
games until they finally made it into Halo 3: ODST
. As mentioned previously, in the novels, the Engineers were never demonstrated to be used for any offensive purposes, essentially being the Covenant's
scientists. To mesh with that lore, in ODST
, Engineers would provide nearby Covenant
forces with an overshield
. However, the Brutes
had covered the Engineers in explosives that would go off if the player got too close to an Engineer or shot at it.