As soon as Mordekaiser activates his W, disengage as he will block the damage you deal to him if you continue to fight him.
Mordekaiser is quite immobile. If you keep the minion wave closer to your side of the map, you will force him to overextend for farm. You can then capitalise on this and get assistance from your Jungler.
Once Mordekaiser hits level 6, his kill pressure in lane grows. Keep this in mind and be wary with your positioning when low as he may try and catch you out with his Ultimate R.
Ward your flanks as Mordekaiser may try to attack from the side in order to get on a high valued target with his Ultimate R.
If Mordekaiser misses his E in a team fight, it wouldn’t be a bad time to focus him down as he will be unable to pull your team in when his E is on cooldown.
Avoid starting major objectives like the Baron or a Dragon when Mordekaiser is nearby as he can take your Jungler into the Shadow Realm with his R which will make taking the objective risky.
At level 6, Mordekaiser’s kill pressure increases. Whenever his Ultimate R is up, you need to play more respectfully.
Mordekaiser has a strong all-in potential as soon as he hits level 3, but before that he can be poked out of the lane. Abuse him as much as possible before he hits level 3.
Team fights are where Mordekaiser can shine. If you’re a carry champion, make sure you invest in a QSS so you can get out of his Ultimate R.