Mord is just one of those champs I have no idea how to lane against. I use ahri, I try to q him but it does zero damage through his constant shield, he just keeps spamming spikes with a bit of spell vamp so I can't go near cs to aa and I'm forced to use Q to farm and lose all of my mana. If I try to fight him, he just completely over powers me because after using my whole kit he only loses about 25% of his health.
You're trying to poke down a champion that has a shield that's constantly refilling? Don't see your problem! 
Seriously though, chances are he started 
and is rushing a 
. That's a massive amount of sustain for a mid laner. You're not poking that down, especially since Morde has no mana to run out of. He's very weak early game, so go all in and be aggressive when you think you can. Once he gets his ult, that's when you know it's time to hang back and take it easy. Feel free to roam at that point, since there's no way he can follow someone with mobility, and even though he can push lane like a champ he can't deal damage to towers well.
Also remember that Morde has 0 cc and 0 mobility, and his AoE abilities tend to push the lane. This is perfect for a quick call to your jungler. When Morde gets ganked, he either burns flash or stands and fights until he dies; no running away for his slow ass.
Keep in mind that any cc that prevents him from casting (silence, stun, charm, taunt) will stop him from regenerating his shield as well, leaving him open for the pain.
Best way to beat an MDK is levels 1-5, he must be punished early and hard, if you let him farm and scale you will lose. He will be so oppressive late game with his sustain and AOE dmg, you'll have to get the whole team to kill him. Get your jg to help out, as he trades fairly well against most laning opponents - but he is a feast or famine champ.
Since I main him,i know what annoys me the most :p Mordekaiser will surely up his E (damages in a cone,just like annie's).if you want to poke him,dont stay near your minions.his passive convert 35% of the damages into a shield if you hit a champion and 17.5% on minions. you understand that if you and your minions get hit,his sustain will be higher. if you want to harass him,poke him from the side of your minions.he will have to choose between hitting minions to shield himself (and not damaging you) or to hit you (but his shield won't be really well charged). Still,What I do to counter that (when i'm mid) is to rotate to have the minions between me and the enemy champ.that requiere WARDS!!!! good luck :)
Poking him is a waste of time. Just focus on farming because his carry potential falls off if he's not fed and during lane he'll just use all his spells to farm anyways.
Before he get his spellvamp he's super vulnerable. If he's maxing his Q, he'll be wasting a lot hp for cs. If he's maxing E then he have to cs in melee range and can be pusnished.
If you didnt shotdownt him early nothing else you can do except you're Riven. Wait till late game when he'll fall behind. Morde has old design and can't be balanced well. His mid game absolutely insane now.
