Frondell joined the Chicago Blackhawks when he was recalled from Djurgardens IF of the Swedish Hockey League on March 24, 2026. He had an assist in his NHL debut that night, a 4-3 win at the New York Islanders.

Frundell, the No. 3 pick in the 2025 NHL Draft, had 28 points (20 goals, eight assists) in 43 regular-season games while leading SHL rookies in goals. He became the fourth 18-year-old in league history to score at least 20 goals in a season, joining Thomas Sandstrom, Markus Naslund and Daniel Sedin. Frondell also helped Sweden win a gold medal at the 2026 IIHF World Junior Championship with eight points (five goals, three assists) in seven games. He was named Best Forward of the tournament and earned All-Star Team honors after leading Sweden in goals and ranking fourth in points.

Frondell was once described by a scout as a "military tank" because of how competitive he is and how hard he plays. He won't shy away from physical play, which was especially impressive from an 18-year-old player skating in Allsvenskan, the second-highest professional league in Sweden. He had 25 points (11 goals, 14 assists) in 29 games of the 2024-25 season and seven points (three goals, four assists) in 16 playoff games to help Djurgarden earn promotion to the SHL for the next season.

Frondell made his Allsvenskan debut as a 16-year-old in 2023-24. He hit a roadblock in 2024-25 when he had surgery that September for an injury to his left knee and played three games before a decision was made to send him to the junior league.

Rather than see it as a demotion, Frondell embraced the opportunity to get his game back on track while still playing against older competition. He had seven points (five goals, two assists) in his 10 games, and scored a goal in his first game back in Allsvenskan against Kalmar on Nov. 12.