McLaren said in a statement on Thursday that the Briton had been appointed president of McLaren IndyCar with immediate effect, reporting to the Formula One team's American principal Zak Brown.
"Fernley will build and lead a technical team entirely focused on the 2019 Indy 500, while helping to evaluate the feasibility of a longer-term McLaren involvement in IndyCar," they said.
Fernley left Force India after the team went into administration in July and were taken over by a consortium led by Canadian billionaire Lawrence Stroll, father of Williams driver Lance.
A close associate of Force India's financially-troubled previous co-owner Vijay Mallya, for whom he effectively ran the team, Fernley has previous involvement in North American racing in a long career in motorsport.
"The 500 is a hell of a challenge and we have incredibly strong competitors to overcome if we’re to be successful. We will need to prepare well for the month of May and that work starts now," he said.
Alonso, 37, is leaving Formula One at the end of this season and is aiming to become only the second driver to complete the 'Triple Crown of Motorsport'.
The Spaniard is a double Monaco Grand Prix winner and also won the Le Mans 24 Hours this year, leaving only victory at Indianapolis between him and matching the late Graham Hill's 1970s achievement.
(Reporting by Alan Baldwin, editing by Toby Davis)
