[4] In addition to his work in television, Schnapp had a supporting role in Steven Spielberg's historical drama Bridge of Spies (2015)[5] and voiced Charlie Brown in the animated film The Peanuts Movie (2015).

[16] Schnapp's parents registered him for an acting program at Star Kidz in Westchester, where he was guided by coach Alyson Isbrandtsen.

[8] Schnapp's professional acting career began when he voiced the lead character, Charlie Brown, in the animated film The Peanuts Movie in 2015.

[16][17] That same year, he made his film debut in the historical drama Bridge of Spies, directed by Steven Spielberg.

[5] His breakthrough came in July 2016, when he began starring as Will Byers in the Netflix science fiction horror television series Stranger Things.

[29] In 2018, Schnapp played the son of a street artist in a short film called The Circle, set during the Great Depression.

[35] Schnapp also voiced the character of Charlie Brown for the video game The Peanuts Movie: Snoopy's Grand Adventure.

[38] In November 2021, he launched To Be Honest (TBH), a sustainability-focused snacking company he co-founded with Elena Guberman and Ba Minuzzi.

[42][43] In 2023, he launched TenderFix, a delivery-only virtual restaurant brand operated by IHOP with a menu of chicken tenders.

[53][54] In January 2024, he posted a TikTok video advocating for peace for both sides and asking for understanding and compassion, stating that his thoughts and beliefs on the war had been misconstrued.