Ash captured Kingler as a Krabby in Mystery At The Lighthouse so he could prove to Misty that it was not a fluke that he captured his other Pokémon. Immediately after the capture, Krabby's Poké Ball teleported away, due to Ash already having six Pokémon in his team. Brock later explained that Krabby was sent to Professor Oak.
Ash decided to use Krabby in the first round of the Pokémon League in Round One - Begin!, reasoning that it would be perfect on a Water battle field, despite the fact that Krabby had never battled before. Krabby managed to defeat Mandi's Exeggutor and the experience it subsequently gained during the battle caused it to evolve into a Kingler.
Kingler then went on to defeat Mandi's Seadra and Golbat, winning the match for Ash. Kingler was also used on the Ice field, where it defeated Pete Pebbleman's Cloyster, before falling to his Arcanine.
+ indicates this Pokémon used this move recently.* - indicates this Pokémon normally can't use this move.
Kingler is Ash's only Pokémon to sweep an entire league match by itself (not counting one on one matches).
Ash's Kingler has used more moves in a single episode than any of Ash's other Pokémon, using eight moves in Round One - Begin.
Ash ordered Kingler to use Hyper Beam against Mandy's Golbat, despite the fact that Kingler could not learn Hyper Beam as a Krabby and had never displayed the ability to use it earlier in the battle, which was against the same opponent who had made it evolve in the first place.
While as a Krabby, Rikako Aikawa who also plays Ash's Lapras and Liza's Charla voices it in both the Japanese and English-language versions. However once it evolved into Kingler, Unshō Ishizuka who plays the Narrator and Professor Oak in the Japanese version took over the voicing for Kingler in both versions of the anime.
It is the only Water-type Pokémon on Ash's team so far he has evolved.