Native to the planet Dorin, like all Kel Dor, Koon was accustomed to breathing in an atmosphere containing little oxygen. Hence, in order to survive on oxygen-rich worlds, a Kel Dor had to wear protective goggles and an antiox face mask which also supplied him with a trace gas found only on his home world. An adept and skilled warrior, Plo came from a long Kel Dor Jedi dynasty, including an uncle and Plo Koon's own niece, Sha Koon. Plo Koon studied the Jedi way under the tutelage of legendary Wookiee Jedi Master Tyvokka.
Kel Dors had a reputation for making quick and clear distinctions between what is right and wrong. Plo Koon was no exception, often taking swift, decisive action with little or no contemplation for repercussive effects. This attitude, combined with Koon's sarcastic wit, often tried the patience of his Jedi Master. In his career, Plo trained many Padawans, including Bultar Swan and the female Trandoshan Lissarkh, who was Koon's apprentice during the Battle of Naboo. Koon was a close friend of Qui-Gon Jinn, and believed that Qui-Gon deserved a seat on the Jedi High Council. He even suggested the idea to Tyvokka, though the elder Jedi disagreed. However, Koon soon came to realize that due to his rebellious nature, Qui-Gon would never find a place on the Council.
Before he could accept his former master Tyvokka's offer to join the Council, Koon accompanied the Wookiee to Troiken, where the Republic was to negotiate with the pirate Iaco Stark. Stark had been stealing bacta supplies from cargo vessels and selling them at lower prices than the Trade Federation could afford to offer them. Rather than resort to military action, the Republic decided to negotiate, with Master Tyvokka as chief diplomat.
However, some factions in the government wanted more decisive action, and they organized an attack on Stark's forces, which caused the negotiations to degenerate into a firefight. Plo Koon watched in disbelief as Tyvokka was killed in the crossfire. In the wake of his Master's death, Koon rallied the forces of the Republic and led them to a miraculous victory in the ensuing conflict, which came to be known as the Stark Hyperspace War. This triumph made Plo Koon a hero of the Republic. He was made a Jedi Master and would hold a life-time seat on the Jedi Council in honor of his Master's dying wish. During the battle Plo befriended a Republic soldier, Jace Dallin, who later spoke of Koon's bravery and humility.
After the Stark Hyperspace War ended, Plo Koon was made a Jedi Master, and accepted a seat on the Jedi Council, where he served for many years. Plo Koon, ever humble, felt there were many others more qualified than he to serve on the Council. Plo was also a close friend of fellow Council member Micah Giett and went on many adventures with him. Their constant verbal sparring made them appear as rivals, but those close to the two Jedi Masters understood that they were indeed good friends.
Giiett and Koon were responsible for the release of the chef Slabba Drewl from the clutches of Corpo the Hutt on Ord Mantell. In a cooking contest with Corpo's kitchen droid MREM-02, Giiett was able to distract the audience while Koon swapped the droid's ingredients.
When judged, Giiett's Squirmer Stew, though average, was enough to win the competition and release Drewl. It was after this mission that Giiett considered the possibility of using two lightsabers.
During the fighting, Koon used a Force technique to conjure and manipulate clouds of dust to cloak the Jedi from their Yinchorri attackers. Due to Koon's effective smokescreen, they were rescued by Adi Gallia, Eeth Koth and Tsui Choi, and soon embarked on a mission to rescue the remaining Jedi. Unfortunately during the conflict Giett was mortally wounded and chose to stay behind to distract the enemy. In honor of his fallen friend, Koon took over the training of his Padawan, Bultar Swan.
In most meetings with the Council, Plo Koon was a silent, enigmatic figure who rarely joined in with debates, but his knowledge was valued by the other Councilors. Plo was present when Qui-Gon brought the young Anakin Skywalker before them, asking to be trained as a Jedi. The Council's members were skeptical of the boy's suitability, and Qui-Gon's usual headstrong attitude did nothing to help matters. Before the Council came to a final decision, Qui-Gon was killed on Naboo, and Koon now had to mourn another friend. He, along with the rest of the Council, went to Naboo for Jinn's funeral.
During the years after the Battle of Naboo, Plo used his telepathic abilities to help Aayla Secura regain her memories following an incident with glitteryll on Ryloth. He also traveled with fellow council members to Malastare to negotiate a truce between the terrorist group known as the Red Iaro and the Lannik royals.
Koon remained on the Jedi Council for the next decade, witnessing a growing public discontent with the galactic government and the Jedi. A splinter group known as the Separatists emerged, led by former Jedi who had now taken the name Count Dooku. At the same time, a group calling itself the People's Inquest began enlisting support for a motion that would have forced the Jedi Order to be more accountable to the government. Plo Koon himself used a Jedi mind trick to disperse a crowd of student protesters who had infiltrated the Jedi Temple.
When an assassination attempt on Senator Padmé Amidala's life caused an uproar in the Senate, Chancellor Palpatine held a meeting with the Jedi and the Loyalist Committee— a meeting which Plo and fellow Council members Mace Windu, Ki-Adi-Mundi and Yoda attended. Plo was also present when Obi-Wan Kenobi contacted the Senators when he uncovered Separatist activity on Geonosis.
Plo was one of the 200-strong task force of Jedi who fought at the Battle of Geonosis in 22 BBY, joining with fellow Jedi Ki-Adi-Mundi, amongst others, in a mission to deactivate the Droid Control Ship on Geonosis. Unfortunately, the Separatists had a backup control device, and Koon and Mundi were captured. They were marched to the execution arena where the surviving Jedi awaited imminent death. Just as the surrounding battle droids readied their blasters, the Jedi escaped with the timely intervention of the Republic Army. Plo Koon led a company of clone troopers into battle on Geonosis, and served as a General in the Grand Army during the Clone Wars. He was among the few to survive the battle of Geonosis.
During the war, he coordinated the Battle of Brentaal IV, advising Shaak Ti and joining her in meditating after the battle. Plo and Adi Gallia worked together for a while during the Clone Wars. They helped Anakin Skywalker and Ki-Adi-Mundi locate Obi-Wan-Kenobi and ARC trooper, Alpha, both of whom had been reported missing in action. During the search, the Jedi participated in a space battle against bounty hunters over Varonat. Also the double act rescued Senator Bail Organa when he was attacked by pirates.