Mordekaiser's single target damage with Mace of Spades is extraordinary and functions great in assassinating squishy targets, what it also helps with is clearing large monsters in the jungle. Despite losing splash damage on his Mace of Spades the ability to do massive damage to blue, red, and gromp more than makes up for the deficit and makes Morde a much more viable Jungle pick than he was before update 5.16.

I've found that Morde is fully capable of clearing your entire jungle often without needing to use your final pot.
Preferred clear;
1. Gromp - Smite
2. Blue
3. Wolves
4. Raptors
5. Red - Smite, 1st pot
6/7. Krugs - last pot
6/7. Scuttle crab

5.18:
The changes to Morde's Creeping Death (W) have increased the base damage, heal, and reintroduced self-cast sacrificing 25hp as the new cast cost. The most important part is the "25% heal against minions" which does not apply to jungle creeps and you will gain the full heal.

These changes to Morde's kit have made his sustain much stronger in the jungle due to the ability to pick up Creeping death earlier and providing increased AoE followed by a heal of around 160hp with a rank 1 W in a camp with at least two monsters.

Now you can't pick up Creeping Death at level 1 for better double jungling or for exp sharing with bot lane since the full exp off morde's W state minions killed, not monsters/units.

5.17:
Starting Mace of spades is already explained above for the clear of gromp and blue, next should be Siphon of Destruction to help clear blue and wolves, at lvl 3 before raptors rank Mace of Spades a second time instead of Creeping Death. (This is before the 5.18 self cast, might be better to get W 3rd at that point.) Usually Creeping Death will be useless in the jungle and that is why I pick it up at level 4 after Krugs.

5.18:
With the new changes and increased sustain morde can now easily clear the full jungle without using his last pot and often may be able to sustain enough to not need either pot.

The new sustain he has early means that it is much easier to gank level 3-4 after a clear with high-full health. Early ganks are still weak against lanes with lots of CC and escape. In order to effectively gank he needs the lane to either be pushed in his favor and/or the champs in lane to apply CC to aid his gank. He is still weak early, but expect to be able to gank much more often than you have been able to in previous patches.

5.17:
Prior to level 4 I wouldn't recommend ganking, I prefer to get at least two ranks in Mace of spades so that his damage in lane is significant and you need Creeping death to help close the gap between you and the enemy in lane.

Morde's early ganks are fairly weak in lanes with little to no CC or against pushed in lanes, when jungling as Morde look out for lanes that are clearly push towards your turrets and make sure that the laner is ready to slow or stop the enemy from escaping.

Devourer on Mordekaiser gives him the necessary attack speed to quickly clear and proc his powerful third Mace of Spades damage spike. To synergize with the phantom hit I prefer to build Titanic Hydra for even more AoE and health which is lacking on AD morde.
Building BotRK or Wit's End also work quite well, but they are more situational.
I have tried using Cinderhulk and Runeglaive but they both came off as slow and their damage is considerably lower unless you build full AP morde with runeglaive, even then the mana pool and regen seems wasted.

Rylai's, Trinity, and Sterrak's Gage are all good items on morde but are somewhat situational for what you need in your circumstance.

Must haves for this build are Titanic Hydra, Devourer, and Deadman's Plate boots and other items are completely optional and dependent on your game, there are no real certainties I would say regarding other items.