Hi! This is my first build... It may not be good. But I'll give it my best shot. Ahem.
As most of you know Pantheon is a great early game champion. And in this build I will show you how to build a great mid Pantheon.

Pantheon

Pros:
Strong AD
He can jump high!
Ranged attack for harassing
Stun
Passive shield can save you
Great early game killing power
15% hp or lower automatic crit to death
Can focus the enemy carry in team fights
Cons:
No way to escape
Squidhy if only built for damage
Not the best dps
CD is a pain in team fights

When i mid with Pantheon i tend to be a jackass and harass with Spear Shot. I stay behind my ranged minions to make sure when the enemy attacks me they gain minion agro. I use spear shot to last hit minions so even if they harass me a lot i get the money and exp I need to get the right items. Before you start to gank you have to make sure you have your Grand Skyfall. Just a habit of mine, jumping out of nowhere and kick the enemy champions ***. I mostly use my Aegis of Zeonia then a follow up of the Heartseeker Strike and to end it a spear throw to the face a boom dead. Thats basically what you need to do in a 1v1.

Many of you know runes are a big part of the game and may have huge impact of whether you lose or win. I get armor pen marks, straight up health quin, dodge seals, and magic resist per level glyphs. This allows me to do a lot of damage from my abilities and getting out of battles without much harm. Also as you probably already know Pantheon is very squishy and needs some kind of way to escape.

I go with a 9 21 masteries. I take every damage reduction mastery I can get. And try to get as much attack and attack speed as possible. This is very important due to the squishy and the not much of a escape mechanism.

As you should know Pantheon has a very good passive. Every 4 attacks gives him a shield that will block a attack. The items I get help me quickly get the shield to protect me from harm. I build berserkers, phantoms, infinity, phantoms, bloodthirsters, and finally a frozen mallet. I infinity for the crit. Bloodthirster for the life steal and attack. And frozen mallet for health and slow. But instead of two phantoms you could get a youmus just for the attack, crit, and the active. On your way back i suggest you get a elixir of fortitude optional of course because of its attack boost and its health booste.

For mid i need the spear throw. I get that as my first ability then I get one of each spell. After I try to max out heartseeker as fast as you can. Then i get spear throw to finish off the champ that run away. Make sure you keep ghost to either run away and chase down that dirty rat bastard that left his teammates. And if your running low on mana just use clarity to make sure u get the kill instead of humiliating yourself and dying because of low mana

As I know from experience Pantheon is very mana hungry in the begining so i get clarity. But as a escape mechanism I use ghost to get the hell out of there.

With this build I have done so many positive scores its hard to count. Many mids have fallen to this build. And with that said I wish you good luck and have fun kicking ***. Thank you for reading this build.