This is well before the ESRB existed, but Manhunter: New York from 1988. It only had Tandy 16 colors, but luckily one of those colors was red. Because there was a lot of blood in it. I was 7.

Pretty sure it was Mortal Kombat 3…I fell for the hype as a kid and it honestly wasn’t really my style.

Yep. Games weren't regulated like that until the '90s.  Fortunately, I never played any of Mystique's terrible Atari games but I did play Larry on a friend's PC back in the day.  And I never even knew about Chiller until much later.

But yeah, Bionic Commando in 1988 was the first game I actually owned with something as graphic as resurrected Hitler's exploding head:

Spoiler!

Love that game; the arcade game is pathetic compared to the excellent NES version.

About a year later, Last Battle on the Genesis was censored in the US because it was actually a Fist of the North Star game in Japan on the Mega Drive:

OP:  So, since I never bought Mortal Kombat or Doom, I guess the first game I actually owned with the "M" rating was Turok on N64.

But the first time I was moved by more mature story events had nothing to do with an "M" rating.  In Phantasy Star 2 on the Genesis, a girl is killed by her own father near the beginning of the game, who then kills himself:

Later, about midway through the game, one of your main party members dies in the game, permanently.  This was almost a decade before Aerith in FF7:

My parents were kinda funny. I wasn't allowed to watch the A-Team growing up but played Leisure Suit Larry on my dad's PC.

And this




Before that we already played war games on C64. Found it:



Now that triggers nostalgia.

Mortal Kombat 2 on MegaDrive. It didn't scare me as a 9 year~ old kid. I thought it was cool with a unique vibe. And it didn't make me violent! I continued to wash + dry dying ants in the sink area attempting to revive them lol.

Grand Theft Auto Vice City on PS2.

Parents didnt care

Doom on SNES, baby.

Pixels the size of LEGO bricks at 15fps, and it was glorious.

I don't think I got to play anything mature till teen - not that age ratings existed before that.
Back in school there were copies of first Larry going around computer class (before what we called "Great Game Extermination").
Earliest game I had at home to play was Wolfenstein 3D.
Don't think I actually bought(/had parents buy me) anything really mature before I was adult, except Police Quest 3. And if you count Double Dragon for violence.

My memory is a little hazy, but I think it was a French erotic visual novel game from the 80s or early 90s, possibly translated from Japanese, that roughly translates to something like "Fuck House"...
...so, it might have been called "Maison du Fuck". I looked it up, couldn't find references.

Same for both. I remember playing Liberty City Stories as well and my mum watched for a bit and saw me kill someone and then shove a chainsaw into the chest of the corpse and she didn't like seeing that but didn't care much.