Alonso retired from Formula 1 at the end of 2018 after a 95 race career over two stints with the English squad.
He won two world championships with Renault, and had a spell with Ferrari, but struggled for competitive machinery in the latter part of his career.
Though he announced his retirement from F1, Alonso has remained involved with McLaren and will compete in the Indianapolis 500 this year for the squad.
His ambassadorial role firms up that relationship, and will likely see the 39-year-old take the wheel of the 2019 MCL34 and play a part in the development of next year's MCL35.
“Becoming a McLaren ambassador is a true honour,” said Alonso.
“It is a special team, and despite the challenges we have endured recently, it remains so. I said before I stopped racing in Formula 1 last year that I see myself with McLaren for a long-time to come, so I am delighted at this new role and the ability to stay closely involved with the team I feel is my spiritual home.
“We have the Indianapolis 500 in May of course, which I am looking forward to immensely, but this is just the beginning of many things we can do together.
“I am particularly passionate about nurturing young talent, whether that's with my own team or helping the new generation of Formula 1 drivers at McLaren unlock their true potential.
“This is important to both the team and myself, so will be an especially rewarding part of my role.”
Throughout the season McLaren has four test days at its disposal, and while two of those must use rookie drivers, Alonso could take part in the remaining two.
According to team boss Zak Brown, Alonso's new role will allow the team to use the two-time world champion's experience in developing its current, and future cars.
“For any race team, having someone of Fernando's class on hand to provide support through his experience is of huge value,” Brown said.
“His insights and perspective will be welcomed by both our drivers and engineers alike, while his stature and character remain highly appealing to our partners and fans.”