Brock obtained Marshtomp as a Mudkip in A Mudkip Mission. Mudkip lived at a pond on Dewford Island where Mudkip were bred for the Pokémon League. These Mudkip were to be distributed to beginning Trainers as starter Pokémon when they got large enough. However, the Mudkip that Brock caught was already mature and it acted as a guardian of the younger and smaller Mudkip. Old Man Swamp, the owner of the pond, felt that the Pokémon League would not want to give a grown-up Mudkip to a beginning Trainer. Brock eventually befriended it after a few failed attempts. Later, Mudkip, Brock, and Brock's other Pokémon, Forretress and Lotad, had to rescue a baby Mudkip from falling over a waterfall. They managed to save it, but right after the ordeal they were attacked by Team Rocket. All of the Mudkip and Brock's Lotad used Water Gun on the ground around the mecha to make it sink into the ground, and Ash's Pikachu finished the job with Thunderbolt. Old Man Swamp later suggested that Mudkip go with Brock, to which it happily agreed.
Throughout the group's journey through Hoenn, Mudkip was usually seen alongside Brock's Lombre. The two were often called out to use a dual-Water Gun attack.
In Turning Over a Nuzleaf!, Mudkip, along with Pikachu, Treecko, Corphish, Torchic, Silcoon, and Lotad, were separated from the group after a Loudred attacked them. Mudkip and Pikachu attempted to lead the group of Pokémon as they searched for their Trainers. However, a group of Nuzleaf began to capture the Pokémon one by one after Corphish damaged the leaf on one of the Nuzleaf's heads. Mudkip and Pikachu were not captured and worked to save the others until Ash and the group arrived.
In Judgment Day!, Mudkip was used in a mock battle alongside Lombre against Ash's Swellow and Corphish to help Jimmy train as a referee. Lombre was quickly pushed into a tough position and Brock had it play dead to give Jimmy an easy judgement. As Jimmy wondered what to do, Ash commanded Swellow to attack Lombre, who retaliated with Razor Leaf. The attack failed to knock out Swellow and it and Mudkip attacked with duel Water Guns, which Corphish blocked. Jimmy declared Corphish unable to battle, though it sprung back up immediately after. His teacher scolded him for the rash judgement and the battle was put to an end.
Kanto Battle Frontier
Brock decided to bring Mudkip with him across the Kanto region during Ash's Battle Frontier challenge.
In A Chip Off the Old Brock, Mudkip evolved into a Marshtomp when it was having a practice battle against Ash's Grovyle. It automatically learned Mud Shot upon evolution due to it becoming part Groundtype. In the same episode, it became smitten with a Flaaffy, much like the way Brock was smitten by its Trainer.
In What I Did For Love!, Brock decided to enter the Gardenia TownPokémon Contest in attempt to win the affections of a Pokémon Breeder named Yuma. Marshtomp was motivated to help its Trainer, so Brock decided to use it in the Battle Round of the Contest. It performed rather well, taking out Mollie's Absol. In the finals it fought against May's Eevee and was able to use its fins to sense Eevee's underground movements and detect exactly where it would come up after using Dig. Despite this Eevee eventually won.
Initially, Mudkip was distrustful of strangers and was very protective of those it cared for, similar to Ash's Bulbasaur. This is shown in its debut appearance where it viewed Brock and the others as intruders of its land. Once Brock gained its trust, it turned into an extremely loyal Pokémon and felt safer around strangers. During its travels with Brock, it was shown to be more of a sensible type and it was seen assisting Pikachu in breaking up and resolving arguments between the other Pokémon. It is also friendly with younger Pokémon. When Mudkip evolved into Marshtomp it retained the same personality but also developed somewhat of a flirtatious side, like its Trainer, as seen in A Chip Off the Old Brock when it fell madly in love with Mariah's Flaaffy.
Marshtomp was often used to help track down Team Rocket when they had stole a Pokémon. It used the fin on its head to detect vibrations in the ground that the criminals made while running away. This ability extended beyond just finding lost Pokémon, as it also used it in A Real Cleffa-Hanger to try to find Misty and Max.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
In the TCG
Brock's Marshtomp was featured in the TCG multiple times. The following is a list of cards named either Brock's Mudkip or Brock's Marshtomp.
Brock's Marshtomp Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.