He raced go-karts from 2000 to 2002 in the minime and junior classes, and won a regional title and also achieved fourth place in the French championship.

Dillmann moved into the Formula Renault Eurocup in 2005, as part of a three-car Prema Powerteam effort alongside Kamui Kobayashi and Patrick Rocha.

He returned to the European series at the start of the 2006 season; again as part of a three-car team, this time with SG Formula, along with Sten Pentus and Carlo van Dam.

[5] He added two further second places at the final meeting of the season in Barcelona, finishing behind eventual series champion Filipe Albuquerque on both occasions.

In the French championship, Dillmann finished in tenth place after taking two consecutive victories late in the season, at Le Mans and Magny-Cours.

Prior to the 2007 season, Dillmann became a member of the Red Bull Junior Team, alongside fellow French driver Jean-Karl Vernay.

[8] With added financial support from Red Bull, Dillmann entered the Formula 3 Euro Series with ASM,[9] joining Romain Grosjean, Nico Hülkenberg and Kamui Kobayashi at the team.

[14] Dillmann returned to the series later in the season at the Nürburgring, with the Jo Zeller Racing team,[15] after they parted with Michael Klein.

After sitting on the sidelines in the first half of the year, Dillmann replaced Kevin Mirocha at HBR Motorsport in the Euro Series; ahead of the Oschersleben meeting of the championship.

Dillmann also contested the final two meetings of the season; he competed at Dijon with Prema Powerteam,[19] and again with HBR Motorsport, at Hockenheim.

[22] At the first meeting of the year at Oschersleben, Dillmann won the second race on-the-road, before being demoted to seventh after a post-race penalty for jumping the start.

[27] Consistent points finishes were the key to Dillmann's second half of the campaign, going on a run of five races without a podium, but his championship lead was slightly reduced by Abt.

Dillmann also contested the Zolder round of the German Formula Three Championship, competing in the Trophy class for older-specification machinery.

At the opening round of the season in Istanbul, Dillmann qualified on pole position for the first race, recording two laps good enough for the top spot.

[42][43] His testing performances enabled him to join the team for the non-championship GP2 Final event held at the Yas Marina Circuit, in support of the Abu Dhabi Grand Prix.

In 2014, although confirmed at Russian Time, the death of the team principal in January had the consequence that he lost his seat; Dillmann contested eight rounds of the championship with Arden International and Caterham, finishing on the podium in the sprint race at Catalunya and achieving the fastest lap in the feature race at the Hungaroring.

[51] Taking two race wins, five pole positions and two fastest laps, Dillmann secured the championship at the final round, seven points ahead of nearest challenger Louis Delétraz.

[52] In April 2017, Dillmann partook in the free practice session with Venturi at the Mexico City ePrix in place of Stéphane Sarrazin.

[53] Later that month, Venturi announced Dillmann would make his racing debut at the Paris ePrix in place of Maro Engel.