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 "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 "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!" He is last seen stuck in an air vent. He was rescued along with the other students.
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.
Arnold has a medium complexion, short orange hair, blue eyes and has freckles. He wears framed glasses. Arnold's clothing often consists of vests, sweaters, slacks and sneakers. He isn't as tall as his other male classmates, roughly the same height as the spies.
- 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")
- He is one of the few characters with a confirmed last name.
- "Stuck in the Middle Ages with You"
- "Spy Gladiators"
- "Queen for a Day"
- "Black Widows"
- "I Want My Mummy"
- "A Spy is Born II"
- "Stark Raving Mad"
- "Morphing Is Sooo 1987"
- "Super Nerd Much?"
- "Super Agent Much?"
- "Arnold The Great"
- "The Suavest Spy"
- "Evil Graduation"
- “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")