It'll probably be Perigrin to help my ass out here, but I'll welcome the surprise of anyone else remembering this.
So I'm fairly certain it was a Mechwarrior game, like 90% certain because I remember killing a couple Catapults in one mission, and the ending of the game had I think a Hunchback in it, both definite Battletech Mechs.
The issue is that this game was on PS1 and I've looked through all of the Mechwarrior games I can find on PS1 and none of them look the same, basically I'm comparing the ending video of them all because that's the part I remember best, although I don't remember if it had multiple endings.
From what I remember it ends with a Mech running down a street, it runs up to a building with like pretty much no defences, I think just a couple sandbags outside, even though it's like the HQ of a Clan or something, I think, fires a missile at it and blows it the **** up, and then I'm fairly sure it was game over.
I barely remember any of this game but I remember loving the **** out of it when I was like 6 and barely being allowed to play it because it was my brothers (who don't remember this game among the hundreds of PS1 games we had) and they thought I'd break it.