Technically, my first was some rigid early 90's Jamis MTB. Might have been a Dakota. I got it sometime around 1996.
But my first REAL MTB (the one that got me into serious trail riding) was an 1994 GT Timberline frame with a Specialized Future Shock (rebranded Mag-21) that I bought used sometime around 1997. I ended up upgrading a bunch on that bike before I bought a Santa Cruz Superlight frame in 2000 and moved all the parts over to that.
I regret not having any pictures of that bike from the few years that it was my primary MTB, or even one of the frame's many incarnations over the years that followed. All I have is this picture of the frame before I donated it to someone a few years ago:
