Wahlgren graduated from the University of Kansas with a Bachelor of Arts in Theater in 1999. She lived in Kansas City, Missouri where she did some radio spots, then moved to Los Angeles in 2000 to work on her acting career. In an interview in Lawrence Journal-World, she said that she was not getting much on-camera work, so she shifted her energies towards voice-over work, and that her experience with dialogue and Shakespeare classes helped a lot towards landing roles that involved various characters, some of which had accents or were from certain time periods.

Wahlgren's next major anime dub role was in Witch Hunter Robin, where she played the title character Robin Sena, a soft-spoken girl who joins a group of witch Hunters, but harbors some magic abilities of her own. Zac Bertschy of Anime News Network said she was "pleasant, subtle and perfect for the role" and wished the supporting voice cast would demonstrate such subtleties. She voiced lead character Lavie Head in the steampunk fantasy series Last Exile. Allen Divers of Anime News Network grouped her with the "better known names of the California voice-acting scene". She voiced title character Sakura Kinomoto in Bang Zoom!'s dubbing of Cardcaptor Sakura Movie 2: The Sealed Card, which Divers praised that "the voice actors did a great job of matching the emotions of the original Japanese voices, including Sakura's trademark expression 'Hoe!'", something that was missing in previous English adaptations. In live-action works, she starred as Tinkerbell in an indie Peter Pan film rendition called Neverland, released in 2003. Her character was described as a "sullen sidekick" and "drugged-out".

Wahlgren's involvement in animation voice-over took off in 2004 when she was cast in a lead role in the Disney/Jetix ensemble action superhero show Super Robot Monkey Team Hyperforce Go! where she voiced Nova. The show ran for four seasons from 2004 to 2006. She would also pick up recurring villain roles such as Samantha Paine aka Magness in A.T.O.M. and Charmcaster in Ben 10, the latter of which would return in the show's sequels and related video games. In 2007, she voiced Tak's friend Jeera in Nickelodeon's computer graphics-animated series Tak and the Power of Juju. She voiced little sister character Suzy Johnson in Disney’s hit series Phineas and Ferb, and participated in the second season of the animated comedy Lil' Bush where she voiced Lil' Hillary and Lil' Condi. She became a regular cast member for Fish Hooks, which premiered in 2010, where she voiced Shellsea and many supporting and guest characters. She landed the role of Tigress in the television adaptation Kung Fu Panda: Legends of Awesomeness; the series premiered in 2011 and has run over four seasons. In 2012, she had the lead role of Allie Underhill in the action series Kaijudo, which spanned 52 episodes, and picked up roles in other cartoon shows such as Gravity Falls, Randy Cunningham: 9th Grade Ninja, and Winx Club. She voiced Meg, the starring love interest in the Disney short film Paperman which screened before Wreck-It Ralph in the theaters in 2012. In an interview with her school's alumni association, she said that she was asked to do the role because of her previous involvements in Bolt and Tangled. Recording for the film took about 30 minutes, mostly of vocal sounds. The short went on to win an Academy Award for Best Animated Short Film. In 2015, she joined the cast of Fairy OddParents as a starring character Chloe Carmichael in its show's tenth season. She continued to voice Jessica and other characters for Rick and Morty which has concluded its third season, and voiced Amanda in the Butch Hartman-produced series Bunsen Is a Beast which aired on Nickelodeon in February 2017. In July 2017, she voiced Dorothy Gale in the animated television series Dorothy and the Wizard of Oz broadcast on Boomerang.

Wahlgren has been involved in a number of video games. Her first voice-over role in video games was in Buffy the Vampire Slayer: Chaos Bleeds where she played Willow Rosenberg. In the James Bond video game From Russia with Love, released in 2005, she voiced a Bond girl that was paired with Sean Connery's original voice. In 2006, she landed leading roles of Ashe in Final Fantasy XII and Shelke in Dirge of Cerberus – Final Fantasy VII. In an interview conducted by UFFSite and RPGSite, she recounts how she received notice of the roles on two consecutive days, and said that the franchise was like "the holy grail of video game acting". She portrayed Ashe with a bit of a regal tone, after the Director encouraged her to do so following a class where she was doing Shakespeare. She describes her character as someone a girl would look up to as "intelligent, diplomatic, and can kick ass and look good doing it". In contrast, Shelkie was desired to be portrayed as very cold and emotionless, which she describes as "cold, but sympathetic." In 2007, she voiced the playable title character in the strategy-RPG Jeanne d'Arc.

Some of her work has garnered recognition, awards and nominations from websites that review anime and animation. Mania.com wrote that Wahlgren's performance as the straight-man character Kagami Hiiragi in Lucky Star was absolute sarcastic perfection, and named her the Best Overlooked Actress for 2008 in their annual Anime Dubbies Awards. Theron Martin liked Celty Sturluson, a headless dullahan character in Durarara!! who uses mostly body language and typed words. In his year-end awards for 2011, he selected Wahlgren the Best English Dub Performance – Female. On the Behind the Voice Actors website, she was selected by the staff as the Voice Actress of the Year in 2012, and was nominated for 2011 and 2013. She received nominations in other categories: Best Female Lead Vocal Performer in an Anime Title for Celty in Durarara!! in 2011, Best Female Lead Vocal Performance in a Television Series – Action / Drama for Allie Underhill in Kaijudo in 2012, Best Female Lead Vocal Performance in a Television Series – Comedy / Musical for Tigress in Kung Fu Panda: Legends of Awesomeness in 2012, Best Female Lead Vocal Performance in an Anime Television Series/OVA for Saber in Fate/zero, Best Female Supporting Vocal Performance in an Anime Movie/Special for Karina Lyle/Blue Rose in Tiger & Bunny: The Beginning in 2013, Best Female Vocal Performance in a Television Series in a Guest Role – Action/Drama for Letta Turmond in Star Wars: The Clone Wars and Best Female Vocal Performance in a Video Game in a Supporting Role for Courtney Collins in Metal Gear Rising: Revengeance.

Wahlgren's debut anime role was Haruko Haruhara in a six-episode OVA series called FLCL. In an interview with Anime Tourist, she said that Haruko was the only character she got to audition for. She depicted her as close to the original dub but with some American interpretation to appeal to the audience, and depending on how she was drawn, she changed her voice range from cartoon and childlike to realistic. In a later review of the series, Bryce Coulter of Mania.com was impressed by her character work: "she definitely gives Haruko that twenty-something, sarcastic and almost punkish persona." In 2018, it was announced she would be voicing in the new FLCL seasons, including as Haruha Raharu in FLCL: Progressive and reprising Haruko in FLCL: Alternative.