In "The Sponge Who Could Fly," SpongeBob accidentally causes accidents for Old Man Jenkins, causing him to change his profession, from Farmer to Sailor, and then to Cannonball, where he shoots SpongeBob down.
In "High Sea Diving," Old Man Jenkins tricks the Bikini Bottomites into believing he is King Neptune and proclaims himself as ruler of the sea. However, when the real King Neptune appears, he is not impressed with the lies Jenkins proclaimed. As punishment, Neptune zaps the deluded old man with his trident, burning him to a crisp.
His name is a character name used in various episodes as a running gag. His name is first mentioned in "Sailor Mouth." The name was originally applied to the elderly green fish, who appears commonly throughout the series, but in other episodes, the name "Old Man Jenkins" has been used for completely different characters, such as a green fish in a wheelchair and a blue fish in overalls. The name is always refered to elderly fish.
Old Man Jenkins was Mr. Krabs' neighbor during his childhood. Mr. Krabs and Plankton ask him to try their burger, and when he does, it kills him. Since the show lacks continuity, he is still shown in newer episodes.