In Minato's Generation he was considered a genius.
The Fourth Hokage was well known as the greatest Hokage of all, already creating the A-Rank Ransengan and the S-Rank Flying God Technique before achieving the rank of Hokage.
He was taught by the Sannin Jiraiya. The two were very close and admired each other, Minato even named Naruto after an alias Jiraiya used for himself in his own autobiography, saying that he and Kushina wanted their son to be just like Jiraiya and they asked him to be Naruto's godfather. Minato would also teach his own group, composed of Kakashi, Obito Uchiha, and Rin.
When Kakashi became a Jonin (During the Third Great Shinobi War, 13 years before the series began) Minato threw a celebration for his pupil. He gives Kakashi a special Kunai that he says should only be thrown in a case of emergency. Obito Uchiha arrived at the celebrations late and without a gift, and he and Kakashi begin to argue. after their fight, Minato tells all of them of a mission to destroy a bridge in the Iwagakure region, therefore hurting their enemies economy.
As they moved through the enemy territory, it became apparent that they were being watched. Kakashi ran off stupidly to test out his new technique, but the enemy cloned himself and Kakashi began attacking blindly. A clone attacked Obito Uchiha, who became paralyzed with fear, but he is saved by Minato. He then goes and saves Kakashi from a sword thrust from his enemy, while placing a teleportation seal on his foot. He gets everybody out of there. He then leaves Kakashi, who was injured by the sword thrust, in the hands of Rin. Using the Flying Thunder God technique, he teleported behind the enemy they fought earlier. Realizing his opponent's power, the attacker give the regret of ignoring the order to flee at the sight of Minato before Minato kills him.
Minato returns to the place that he left his team, he tells Kakashi to not use Chidori, because it leaves him not enough sight to repel a counter attack. He then tells Obito Uchiha not to speak without thinking.
That night, Obito Uchiha comes to Minato and asks why Kakashi is the way he is. Minato tells him about Sakumo Hatake, Kakashi's father. He was a famous and powerful ninja who was on par with the legendary Sannin. On a mission one day, Sakumo's team were capture, and instead of completing the mission, he went back and saved them. Because of this decision, Konoha suffered greatly and Sakumo was driven by this failure to kill himself. He then tells Obito that from that day forward, Kakashi adopted a strict mentality of always completing his mission, hiding his emotions and only focusing on the rules. Minato then asks Obito Uchiha to try and understand Kakashi.
The next day, Minato put everyone under the leadership of Kakashi while he went to the front of battle. When there, he gave soldiers dozens of Kunai with his teleportation seals and asked them to throw all of the kunai at their enemy, he then teleported to each kunai and single-handedly defeated the enemy army before they realized what was happening.
Somewhere in his life Minato found time to have a baby. He and Kushina Uzumaki had the baby known Naruto Uzumaki. He was named after the heroic main character in one of Jiraiya's books.
Kushina was in reality the Kyuubi's host 16 years ago. Her pregnancy was attempted to be kept secret, and on the night of her birth, she was taken to a secret location guarded by a seal and some Anbu. Madara appeared, dispatched the Anbu and passed through the barrier. He knew that the seal containing the Kyuubi would weaken during labor and attempted to get the Kyuubi. He took baby Naruto and tried to bargain Naruto's life for the Kyuubi, he threw Naruto in the air and was about to stab him but Minato saved him only to realize that Madara placed explosive seals on Naruto's blanket and quickly got Naruto out of the blanket and fled from the hut which blew up, and Kushina was taken by Madara. Minato placed his Flying Thunder God on Kushina but didn't get there before the Kyuubi was released, Kushina however survived because of the Uzumakis' strong life force. Minato brought her back to their home with Naruto and went back to stop Madara.
Madara arrived at the edge of the Leaf village and summoned the Kyuubi there and it started going berserk. Minato appeared on the Hokage monument and transported the Kyuubi's chakra blast, Madara tried to sneak up on him but Minato reacted instantly and tried to stab Madara, but it passed through him and Madara tried to warp Minato away, but he teleported to safety. Madara followed after where he revealed long chains shackled to his wrists and charged at Minato, he went through Minato and wrapped Minato in the chains, but Minato teleported away. Madara and Minato then charged at each other again, this time Minato threw a kunai which went through Madara's face, but just as Madara grabbed Minato to warp him away, Minato teleported to the kunai he threw and slammed Madara from behind with a Rasengan.
Madara survived the Rasengan and leaped away, with his left arm injured. Unknown to him is that Minato placed a Flying Thunder God seal on his body and instantly stabbed him and placed another seal on Madara's body nullifying his control over the Kyuubi. Madara left promising that the Kyuubi will be his again, and that the world will bow to him. Minato raced back to Konoha in time to stall the Kyuubi from using a chakra blast by summoning Gamabunta to attack it, Minato then teleported it away to Kushina's and Naruto's location. Kushina used her chakra chains to bind the Kyuubi and wanted to drag it back into her and die, but Minato remembered Jiraiya's words about a destined child and a revolution, so Minato believed that Naruto would save the world from Madara and used the Reaper Death Seal to place half of the Kyuubi's chakra in Naruto and the other half in himself, and to seal a little bit of himself and Kushina in Naruto so that they could all see each other one day.
Minato placed a teleportation seal on Kushina. When Kushina was in danger Minato would appear ready to protect her.
He created the "Rasengan" where he forms an intense ball of drilling chakra in the palm of the hand; this technique is actually incomplete since he intended on fusing his own elemental affinity with it before he died. "Flying Thunder God" were he throws a kunai with a seal on the handle, he is then able to instantly appear wherever the kunai is. He's also capable of placing these seals not just on kunais but on other items and living beings, allowing him to teleport wherever that seal was placed. "Reaper Death Seal" where he summons the Japanese Death god, which in return for user's life, would steal the opponent's soul. And an unnamed time-space barrier, allowing him to transport the opponent's attack somewhere else.
Like his sensei Jiraiya, Minato is capable of summoning frogs/toads.
Full List of Jutsu:
Eight Trigrams Sealing Style
Flying Thunder God
Four Symbols Seal
Reaper Death Seal
Summoning Technique: Toads