Hadjar's father, Yassine Hadjar, is a quantum mechanics researcher who also served as his kart mechanic. Named after Isaac Newton, Hadjar took up karting aged seven after watching the Pixar film Cars. He progressed through French F4 — finishing third in 2020 as the highest-placed French driver — and the Formula Regional European Championship, where he claimed multiple victories in 2021 with R-ace GP before finishing fifth overall. He joined Hitech Grand Prix for the Formula Regional Asian Championship and FIA Formula 3 in 2022, winning races in both categories and finishing fourth in F3. He joined the Red Bull Junior Team in 2022.
Hadjar entered Formula 2 in 2023 with Hitech Grand Prix alongside fellow Red Bull junior Jak Crawford. His rookie season was hindered by car reliability issues and his own inconsistency in qualifying, yet delivered moments that underlined his potential.
He demonstrated strong race pace throughout. A mechanical failure while leading the Monaco sprint race was emblematic of his season's frustrations. His best results came at the season finale in Yas Marina, where he ran second in the sprint's opening laps and scored eighth in the feature, ending with two double-points finishes. A first-to-victory at Zandvoort in the sprint was nullified — the race was abandoned after less than two laps under a rain shower following a multi-car accident, so no championship points were awarded.
Hadjar finished fourteenth in the 2023 standings with 55 points, two behind Crawford. The gap to the front — and the reliability losses — prompted his team change for 2024.
Hadjar switched to Campos Racing for 2024, partnering fellow Red Bull junior Pepe Marti. The season became one of the most dramatic championship contests in recent Formula 2 history, with Hadjar and Gabriel Bortoleto trading the lead across seventeen rounds.
His Melbourne sprint saw him win his first Formula 2 race on the road, before a ten-second penalty for contact with teammates demoted him to sixth. The feature race at the same circuit produced his first clean victory, after taking the lead following a slow pit stop by Oliver Bearman and holding off Bortoleto in the final laps. French newspaper Le Parisien reported the result under the headline "le Petit Prost" — a reference to four-time World Champion Alain Prost, who is also from Paris, citing Hadjar's composed, meticulous race management style.
A run of successive wins at Imola, Silverstone, and Belgium gave Hadjar a 36-point lead over Bortoleto going into the summer break. The second half of the season became a study in pressure and misfortune. He missed scoring at Monza despite qualifying on the front row, and crashed out from qualifying at Baku after his brakes overheated — starting twentieth in both races and failing to score. Going into Qatar with half a point separating him and Bortoleto, Hadjar passed Bearman for the sprint lead before his tyres faded and he was repassed, eventually spinning to fourth. He finished second in the feature, closing the gap to half a point ahead of the finale.
At the Yas Marina finale, Hadjar qualified fourth and finished fifth in the sprint to enter the decisive feature race with Bortoleto four points ahead. He stalled on the grid at the start of the feature race — the championship-deciding moment. Bortoleto finished second and claimed the title; Hadjar was classified ninth. He ended the season runner-up with 192 points, four wins, eight podiums, one pole position, and one fastest lap.
The 2024 Formula 2 title battle between Hadjar and Bortoleto was settled by margins of less than one point on multiple occasions during the season. Hadjar's consistency in the first half — particularly his three successive feature race victories at Imola, Silverstone, and Belgium — placed him in a dominant position that the Monza and Baku disasters dismantled. His qualifying pace, often among the fastest in the field, earned him the Red Bull tag of "le Petit Prost" independently of the Le Parisien coinage, reflecting technical precision as well as outright speed.
Hadjar was promoted to Racing Bulls for 2025 alongside Yuki Tsunoda, scoring his maiden Formula One podium at the Dutch Grand Prix that year as the fifth-youngest podium finisher in Formula One history. He was elevated to the senior Red Bull team for 2026 to partner Max Verstappen.