Ilott will drive its No. 6 Chevrolet-powered entry in place of David Malukas, who is recovering from surgery on his left wrist.
The 25-year-old Briton has already subbed for Malukas – who suffered the injury while mountain biking – in pre-season testing at Homestead-Miami Speedway.
Ilott has made 36 IndyCar starts with Juncos Hollinger Racing over the last three seasons, including five top-10 finishes. He also has experience in Formula 1 as he was a test driver for Ferrari and a reserve driver for Alfa Romeo (now Sauber) in 2021.
“Obviously I feel sorry for David and the team,” Ilott said. “It’s not easy to be in this position especially just ahead of the start of the season. I wish him the best and a speedy recovery.”
The deal is a one-off, as in a statement Arrow McLaren says it “continues to evaluate options for its driver lineup for future tests and races as David recovers”.
With that, though, Ilott mentioned there is not a conflict to participate in IndyCar's non-points exhibition at The Thermal Club on March 24, but his status for the event is unknown. Additionally, when asked during the media availability by Motorsport.com about the number of IndyCar events he could do without it impeding commitments in WEC, he responded with 11-12 races "if I had to."
Ilott is competing in the 2024 World Endurance Championship racing for Jota Sport in the Hypercar category, and scored a podium finish in the season-opening race at Qatar on Saturday in its Porsche 963.
He lost his IndyCar ride at JHR to Romain Grosjean, following the ex-F1 racer’s split with Andretti Global last year. The team had already re-signed Agustin Canapino, the 33-year-old former Argentinian Touring Car sensation, which meant Ilott had to make way.
"I was expecting to stay in IndyCar, that was the original plan, and circumstances changed as they sometimes do in life,” Ilott said of the situation at the time.
He finished 20th and 16th in points during full-time campaigns with Juncos, which explained of its decision that ‘both parties have mutually decided to part ways, effective immediately’ and expressed its ‘sincere appreciation for his contributions over the last two years’.
He scored career-best fifth place finishes with the team in 2023, at the St. Petersburg street race, where he will race with McLaren this weekend, and the Laguna Seca finale.
