Hello, I'm andrewrox20, and this is my support Mantheon Guide.
Mantheon as a support is often seen to be a troll pick by many of your teammates, but your adc will soon realize the true strength of Support Mantheon.

Pros

Very Strong Early Game
Targeted Stun and Poke
Very good for a kill-lane
Can build tanky while still dishing out damage
Semi-global presence with his ultimate
He's a baker

Cons

No way of Healing his ADC
No way of buffing/steroiding his ADC
Falls of Late game, damage wise
No escapes, beside his ultimate which has a long start up time
Rediculed for his lack of pants

I always take flash because of it's incredible utility. With this, you instantaneously teleport towards your cursor, taking you over walls, through bushes, and anything else in your path. This is superior to ghost in every way for both escaping and for chasing.

For the Second Spell, there are several options:

Exhaust is the other Summoner Spell that I usually run. With it, you can effectively shut down the enemy adc, making your hail-mary stun bursts on their adc even deadlier. Your stun, combo'd with your burst and then exhaust is pretty much a kill for your adc. Late game, this is still powerful, because it still shuts down its target with a slow.

Clairvoyance is an option as well, if your team already has an exhaust in the top lane or in the jungle. This spell gives you incredible Utility, allowing you to scout enemy jungle buffs, baron nashor, and dragon with just a single click. It's short cooldown is great, allowing you to use it as a long range face-checker for suspicious bushes.

The Final Option is Heal. heal is great on Pantheon, because he doesn't have a heal built into his kit. This can easily turn drawn out lane battles around, and also provides your lane with a sustain advantage over the enemies.

This is what I believe to be the best support Mantheon skill sequence. Early game, what a support mantheon should focus on is constantly harassing the enemy lane with his poke, emerging from a bush to chuck his manliness, and then to walk away. His Q (Spear Shot) is a strong poke which scales nicely when leveled, so that's why I max it first, plus it is targeted, making it easier to hit than say Nidalee's inferior skill-shot spears. I put one point in his W for the stun. Sometimes, if the enemy lane is squishy, you can go all-in at level 2 with your classic Q-W-Auto Attack-Q combo. This can surprise your opponent, and the stun will give your adc time to lay down the damage. I take one point in E at level 4 to improve his burst damage, then go on to max Q.
I max E Next for improved burst, and w is maxed last because leveling it up doesn't improve the stun. Of Course, I take R whenever I can.

I play Pantheon as a tanky utility support, similar to Leona in some ways. His early-game damage is naturally great, so i don't build damage early and focus on building support. I rush Philosopher's stone to improve my sustain and my gold income, then proceed to rush Sightstone, and then Ruby Sightstone to provide vision for my team.
After that, I upgrade my Philosopher's stone to Shurelya's Reverie, to provide even more tankiness. By this point, your damage will start falling off, so your goal is to stun the enemy adc, soak up the damage, and watch your adc tear apart the enemy. Shurelya's active is great for chasing near-dead enemies and also for retreating from an enemy jungle gank. After Shurelya, I build into Aegis of the Legion, then into Locket of the Iron Solari. This item is great because it provides even more tankiness and sustain, making you a meatshield for your adc. This item scales great into the late game because of it's active: It shields you and your teammates from damage. You can Grand Skyfall into a fight, stun the highest priority target, and then use this item when the fight truly breaks out.
For boots, I usually choose the Boots of Mobility, becaue it provides an insane amount of movement speed, which allows you to roam around, stunning people left and right. It also lets you react and position yourself quickly for a mid Grand Skyfall gank. However, if the enemy team is full of CC (Veigar, Sejuani, Sona, Alistar) or has a heavy AP comp (Akali/Mordekaiser Top, Nidalee Support, etc), Mercury Treads are an excellent choice. They provide you with Magic Resist to mitigate some of their AP poke, and also with valuable tenacity which will help you get out of hard cc sooner.
The rest of the Items are situational. I find that building the Frozen Mallet next is great, because it provides its "Icy" Passive, which will slow your enemies and allow your adc to kite them and finish the job. It also provides 700 health which will make you last longer in fights. If you want to build offensively, Zeke's Herald is a great choice, because it provides you with some health for tankiness, some much-needed life-steal for sustain, and an aura that provides attack damage to all nearby allies. This is great and can really boost your team in teamfights, and your adc in general. The Black Cleaver is another option, it's if you decide to build a bit more selfishly, because it doesn't provide an aura like Zeke's herald, but provides a great amount of health and attack damage for yourself. It also shreds your opponent's armor, which can lead to destruction by your adc.
If you decide to build tankier, the best option for you is to build the Sunfire Cape. It provides you with a massive amount of health and armor, and also gives you a strong passive that makes you burn all nearby enemies. Since you will likely Skyfall into a teamfight, this passive is great and will do surprising amounts of damage to the enemy team. This in conjunction with your Locket and Frozen mallet will make you very hard to kill.

As Mantheon, the baker support, you want to be super aggressive in the early game. Head out with your explorer's ward and ward up the enemy bushes, and then once you hit level 2, spear and stun them. The enemy bot lane will probably be caught by surprise, allowing your adc free damage. If you play like a kill-lane, your adc will snowball, and you will dominate the bot lane. The key is to be aggressive. Once you see their adc, or even their support out of position just a little bit, don't hesitate to throw a spear at them, pounce on them, throw your crotch out in a cone, and then spear them down while they flee.
Early Game, if you don't see good opportunities, to stun them with your shield, take every opportunity to poke them with your Q. Hide in the bushes, take a step out, chuck a spear, and then retreat. Rinse and repeat. This will get very annoying very soon for your enemies, and it will do quite a bit of damage. Soon, if they get pissed enough, they will overextend trying to catch you off guard, and that's when you all-in with your stun.
Mid-Game, just ward up summoner's rift, and follow your adc everywhere, serving as his/her personal bodyguard and tank. When you get your Ultimate, and your lane is pushed, feel free to Grand Skyfall into mid-lane, stun the champ, and give your mid-laner an easy kill. You can also use your Ultimate as a short ranged teleport, if you're trying to defend your tower from destruction.
Late game, when you have your tanky items, you will become a utility baker. Use your ultimate to leap into a fight, stun the most dangerous opponent, burst them down with your E, and then proceed to soak up damage for your team. The best combo you have late game is Ultimate-W-E-AutoAttack-Q. Even with your tanky build, this burst combo should bring down most champs to half health if you land every blow. Your stun is powerful late game, because it is targeted and will bring you closer in on the action. Don't forget your passive; You ignore the next autoattack, which makes you even tankier. You are great for initiating fights with your R and your W, but don't walk into a fight your team can't win.

That's all folks, thanks for reading my first guide, and I wish you luck on Summoner's Rift with Support Mantheon!
Don't forget, poke, stun, profit.