I use three mastery builds for Katarina:
First one is 21/9/0 (Aggressive Path):
Offense Mastery Tree is overpowered at the moment. Taking improved spell pen. ignite, + 3 attack damage, %5 ability power increase in the offence and taking armor/magic resistance according to my lane matchup and Durability + Veteran's Scars makes Katarina really really good. (Q:Why +3 Attack Damage instead of +3 Ability Power? A:Because your Attack Damage ratio's are higher than your Ability Power ratio's. + 3 Attack Damage gives you 2.5 damage on Bouncing Blade and 1.7 damage on Death Lotus where as +3 Ability Power only gives 0.9 damage on Bouncing Blade and 0.9 damage on Death Lotus.
You will take one point in this to get the Ignite's +5 attack damage and +5 ability power bonus. Both AP and AD works really well with Katarina.
+2 Attack Damage is really good at early levels. Increasing your damage with Bouncing Blade and Death Lotus, as well as making last hitting easier. You don't need the 3rd AD, because +2 give 2 damage to Bouncing Blade, and +3 gives 2 damage too.
+4 Ap is really good for your Shunpo. Try picking it, if you are going 21 offense.
You are only getting this for Arcane Knowledge.
This is one of the, if not the best mastery for Katarina. Makes you hit much much more than you normally would.
Attack Damage per Level works exceptionally well with Katarina do to her great Attack Damage scalings. The reason why I take Deadliness over Blast is that, Deadliness gives you higher damage on Bouncing Blades and Death Lotus. Since our main damage Source is Death Lotus, I take Deadliness. It also gives you higher damage on your auto-attacks. This may seem redundant but +9 attack damage can decide whether you will secure a kill or not.
Since this skill also works really well with Katarina, I will include it in this list. Like I explained earlier I find Deadliness more useful for Katarina. However you can get Blast over Archmage. At level 18, if you have over 340 Ability Power, than Archmage will give you higher bonus Ability Power than Blast does. If you buy Deathcap, you will most likely exceep 360 Ability Power. So if you will not buy Deathcap, you may take Blast, and it will provide you a little more than Archmage does.
Don't get it, waste of 3 points. Get +3 Ap instead.
This combined with Rabadon's Deathcap gives you around 160 more Ability Power (assuming you have around 450 AP with your full buid, without adding the Archmage or Deathcap Bonuses)than you normally have. If you finish your build you will you will have 450 Raw AP. With the Deathcap and Archmage bonuses, this number goes up to 614. Perfect mastery for Kat.
Probably the second best mastery for Katarina. This increases your damage output significantly. Makes you able to burst people even faster than you normally would.
Gives you free +6 Magic Resist. Get this if you are going against an AP champion.
Gives you free +6 Armor. Get this if you are going against an AD champion
Free HP ? I'll take it. (You should too!)
Katarina starts off with 478 HP, with this mastery, you increase it by 30. This can help you in lane when fighting 1 on 1 or can help you escape some ganks with very low HP.
Get it if you are going 21 in defense. Reduces damage done to you, especcialy significant in laning phase. Very good against Lulu, her Pixie does 1 damage per tick.
Increases your level 1 movespeed. Great stuff.
Bread and butter. Makes you earn 49 gold for each kill/assist.
Free extra hp is always good. Also the cc reduction is very nice.
Against Physical Damage (Eg.
Against Magical Damage (Eg.
The second one is 9/21/0 (Defensive Path)
This is a more tanky mastery setup. Since the Defense Mastery Tree is equally buffed as the offence tree, this Mastery Tree is completely viable on Katarina. Also the Mercenary mastery synergizes really well with Katarina's passive. Mercenary gives you 24 gold for each champion kill or assist and Katarina's passive gives 25 gold for each champion kill or assist, so for each kill or assist you get 49 extra gold. Since Katarina usually gets a lot of kills, the Mercenary mastery is really effective, which makes the defensive path stimulating. I have been using this mastery tree a lot in my last couple of games, and I would suggest testing it out. Nowadays, I use this mastery setup, primarily.
The third one is 27/3/0 (Over Aggressive Path)
This mastery setup is just for the damage it provides. It provides little to non tankyness. Only use this setup if you are going to have an easy lane match-up or if you feel really comfortable using Katarina.
YOU SHOULD NEVER GET ANYTHING FROM THE UTILITY TREE
You don't need cooldown because of your passive, you don't need reduced death time, mana regen. Only thing you need from the utility tree is the improved flash if you are using flash as your primary summoner spell. In that case, you can put one point in utility.