The monsters of season one all serve Bowser and are summoned by Bowser's son Junior using his magic paintbrush.
The Cyber Goom was the first monster that the Mario Rangers fought and had a cyborg/chestnut theme. He wore silver armour, had a mechanical left arm, mechanical left eye, mechanical legs and wielded a laser rifle. He led a legion of Koopsters on his attack on Sommerset and the attack was successful until the new Mario Rangers arrived. After easily destroying his Koopsters they attacked him and he was destroyed when the Rangers used their Ultra-Cannon against him. However he was enlarged by Kammy and attacked the city again. However the Rangers called in their Zords, fought him for a while and then defeated him by creating the Megazord. He was later shown during the Halloween special, being assimilated by the Smithy Empire in Season Three and again during the Series Finale.
The Wicked Wiggler was at first a caterpillar themed monster and then a butterfly themed monster. It had a yellow segmented body with six red arms, a large nose and a flower in its head. It attacked the Mario Rangers briefly before running off and hiding. After this it became a chrysalis and mutated into a butterfly like creature. In this new form he was covered in a yellow fuzz, lost two of his arms and his nose (which was replaced by a proboscis), grew compound eyes, wings and antennae. After mutating he became much stronger and faster and whenever angry turned red and lost the flower on his head. He lead an air raid on Sommerset with a squad of flying Koopsters but was shot down. His rage turned out to be his downfall as the Rangers incited him into such a rage that he became angry enough to explode (which he mentions as being quite common among his species). When he hrew larger the Last Ranger taught the Rangers about the aerial mode of the Megazord and he was defeated. Later appeared during the Series Finale.
The Fishin' Cheep was a fish themed monster who fought the Rangers atop a giant Mesa with a lake. He had red scales, wore a black vest, had fin like appendages and used a high tech fishing rod. He attempted to fight the Rangers by hiding in a lake and manipulating the water and trying to drown the Rangers. However before he could he was shot in the back with a harpoon by Captain Blooper. He and his wife later appear with Captain Blooper and the Sushi Knight trying to find an incredibly destructive Zord known as Unagi.
Captain Blooper was a squid/pirate themed monster. He had a squid like head which was surrounded by tentacles, has a golden hook for a right hand, a tentacle like left hand and a peg leg. He appeared in the same episode that the Fishin' Cheep appeared in, where he shot him in the back with a harpoon launcher. He then attempted to do the same to the Mario Rangers but was knocked off the Mesa they were on by the Ultra-Cannon shortly after he killed the Fishin' Cheep. However he survived the fall and escaped, only to return later with a navy of flying ships. The Rangers managed to get aboard and fought him again where he was fried to a crisp by a flamethrower.
The Sonic Booom was a ghost/sound themed monster. His head was spherical and had two stubby arms, fangs and blue eyes, while his body was mechanical and had two arms with foghorns attached to the shoulders, speakers seven speakers built into the chest and legs with clawed feet. He attempted to crash a rock concert, which the Rangers happened to be at by deafening everyone using sonic devices. The Rangers however quickly dispatched the Koopsters he had with him and destroyed his body. His head however attempted to escape by flying away but was destroyed by a blast from the Ultra-Cannon. Later appeared during the Halloween special and during the Series Finale.
The Bounty Bomb was a bomb/bounty hunter themed monster who was hired by Bowser to destroy the Mario Rangers. He wore red and black armour and had a round black helmet with a large visor, he wielded two small laser pistols. Bounty Bomb began by observing them from a distance and then began stalking them before finally attacking them. Having watched them he was aware of all their battle moves and easily defeated them and was about to finish them off before the Last Ranger appeared and revealed to Bounty Bomb how Bowser's army had devastated his homeland. Angered by this he prepared to go to Bowser's castle to fight him only to be killed by Ludwig, who had been secretly watching the battle.
Mecha-Mole was a mole/cyborg themed monster. His lower body was replaced by tank treads, his torso was heavily armoured, he had two cannons located on his back and two machine guns built into each wrist. Using his heavy weaponry he planned to destroy a large part of the suburbs and replace it with a large weapons manufactoring plant. He attempted to plant dynamite throughout the suburbs but it was stolen by the Mario Rangers and hidden several kilomotres away. Once he found out that his plan had failed he attacked the Rangers and due to his heavy armour had the advantage. However they found out that due to his species living underground and only exposed to dim light he had to use mechanical eyes to see without damaging his eyesight. They managed to short out his mechanical eyes and blind his real ones. After this they succesfully destroyed him with the Ultra-Cannon.
Spannagon was a tool/lizard themed monster. He had purple scales, a wrench like head, snake like eyes, a mechanical tail, a three clawed left hand, a reciprocating saw for his right hand and green armour with four missiles built into the chest. He attacked several building sights and destroyed heavy machinery before turning his sights on the Mario Rangers. He seperated them first by throwing a steel beam at them before attacking them one by one. The Rangers were weakend, but not defeated and joined up to fight him. Once again Spannagon fought, and won. However the Red Ranger managed to work up enough strength to continue to fight Spannagon. The two dueled across the buildings frame until they reached the edge of a beam where the Red Ranger managed to kick Spannagon off the building and into a cement mixer. Despite being incapcitated, the rangers went ahead and destroyed him anyway with the Ultra-Cannon.
Skeeter was a beetle themed monster. He had the head of a large water beetle, complete with legs that often moved about. His body was covered in chitin and so he lacked armour and he also had cup like feet which allowed him to walk on water. He carried a staff that could fire beams of ice. He appeared at a lake and froze everyone that was there, but left the lake untouched. When the Mario Rangers arrived, he skimmed out to the middle of the lake and fired blasts of ice. However the Rangers used the Ultra-Cannon to knock the staff from his hand and send Skeeter falling into the water. As he attempted to get up, his staff struck the water and trapped him under the lake. The Rangers destroyed his staff and used the Ultra-Cannon to boil the water in the lake, which killed Skeeter. The resulting steam flash-thawed everyone at the lake.
General Snifit was an army themed monster. He wore white armour, had clawed gauntlets and wore a mask with an extended nozzle over the face. Over his armour he wore a red robe which covered everything but his face. He was capable of shooting lasers, fire and ice beams from his mouth. He was the newest member of the Snifit High council, an organization which held complete control over the Shy Soldier army. They worked with Bowser before he was trapped in a volcano, and while he was gone they held control over their Home Dimension. He was summoned by Bowser in order to help him conquer the Earth and destroy the Mario Rangers. He brought with him his Shy Soldiers and several assault vehicles. General Snifit led the attack himself. The Mario Rangers attacked the SHy Soldiers, but their were too many of them. The Mario Rangers managed to find General Snifit and engaged him in battle. During the battle, Snifit accidently froze a small portion of his army and at least one mech. He was defeated when Jenny shoved a rock in his nozzle while he was cgharging his laser, the pressure built up until his head exploded. Without a leader the army retreated back to their home dimension.