Arnold is introduced in "Spy Gladiators" where Alex calls him to ask if Donna had forgotten about the fight, but he is selling tickets for people who want to watch them fight and says that things are just heating up. In "Queen for a Day" he offers to help Clover with her homecoming queen campaign on the condition that she date him for a week, but she declines. Clover later accepts his offer, changing the conditions to be his girlfriend for a day. With Arnold launching a full media campaign, he helps Clover win the contest, and leaves holding hands with Clover (although Clover signs for 'help' behind her back). In the A Spy is Born arc, when Sam is recognized as the most popular girl in school, she receives a letter from a secret admirer whose initials are AJ. In "A Spy is Born II" it is revealed that the AJ stands for Arnold Jackson, and Sam is left to honor his wish of going on a date with her.
In the episode "Super Nerd Much?", Arnold accidentally acquires the Cat's Eye Emerald, a gemstone that allows him to steal the "coolness" from other people, turning them into nerds. The power quickly gets to his head, and he turns into a supervillain intent on draining the coolness from everyone in Beverly Hills, presumably as subconscious revenge for all the mistreatment he himself received over the years. After a fight, he was eventually defeated by the girls and the villainous magician Dash Dawson, who originally stole the ring. His memory is erased by WOOHP, after which he has a few residual memories that he writes off as part of a dream. In the episode "Arnold The Great", he aspires to be a superhero and dresses up as the comic book character "Admiral Admirable". After being rejected by the girls, he is recruited by Geraldine Husk who gives him a utility belt that gives him superhero powers. He then captures the girls but is betrayed by Geraldine as a disposable villain. He grabs onto Geraldine so that the girls defeat her.
In "Evil Graduation" of Season 5, Arnold gives the valedictorian speech where he lashes out at the senior class, telling them off for mistreating him over the years and left with the words "Goodbye, and good riddance!"
Arnold is highly intelligent but socially inept, and tends to be a bit unpleasant. In some episodes, he acts as a sycophant and minion of Mandy's, while in other episodes he gets teased by her. He mostly acts as an (unintentional) obstacle to the girls and their objectives.
- In "Stuck in the Middle Ages with You" he wins second place in the Beverly Hills High Halloween party costume contest as the periodic table.
- In "I Want My Mummy" he wins the Beverly Hills High Most Charitable Teen contest after Mandy is disqualified.
- In the episode "Morphing Is Sooo 1987", a Scamlar clone of Arnold is created after some of the liquid metal falls onto him.
- At the end of the episode "Sis-KaBOOM-Bah!" in Season 4, he managed to defeat Clover and Mandy in a contest for head cheerleader.
- Arnold is a member of the MIG (Mandy Is Great) Club at Beverly Hills High. He helps Clover become a member. ("W.O.W.")
- Arnold announces the winner of the Most Photogenic in the Beverly Hills High yearbook. ("The Dream Teens")
- "Arnold The Great"
- "A Spy is Born II"
- "Black Widows"
- "Evil Graduation"
- "I Want My Mummy"
- "Morphing Is Sooo 1987"
- "Queen for a Day"
- "Spy Gladiators"
- "Stark Raving Mad"
- "Stuck in the Middle Ages with You"
- "Super Agent Much?"
- "Super Nerd Much?"
- "The Suavest Spy"
- "You guys still think I'm a nerd?" ("Super Nerd Much?")
- "Let me be the first to say, goodbye and good riddance! From now on, you'll have to find someone else to do your homework and wear your "Kick Me" signs, cause ol' Arnold is movin' up and movin' out!" ("Evil Graduation")