The Monegasque waved to the capacity Circuit de Catalunya crowd but from then on could only watch his 19-point advantage evaporate.
“I had no indications before then. It just broke and lost the power completely,” he told Sky Sports television.
Verstappen also suffered intermittent problems with his car’s drag reduction system (DRS), leading to plenty of cursing over the radio, and he went off at turn four on lap eight.
“I went off when I had a lot of tailwind and lost the rear. My DRS was not always working. That made it very tough but we managed use strategy to get ahead,” he said.
“I tried to stay focused. It is not nice when stuff like that happens. I am very happy to win and happy for Checo (Perez). It is a great result for the team.”
Seven-times world champion Lewis Hamilton was fifth for Mercedes with a fine recovery drive after an opening lap collision with Kevin Magnussen’s Haas dropped him to 19th.
He was passed by Sainz on the closing laps after Mercedes warned of a potentially terminal cooling issue.
“A race like that is like a win. It actually feels better than a win when you’ve come from so far back and through so much adversity,” said Hamilton.
Alfa Romeo’s Valtteri Bottas was sixth, ahead of Alpine’s Esteban Ocon and McLaren’s Lando Norris in eighth.
Norris revealed afterwards that he had been suffering tonsillitis and said it was that “one of the hardest races I’ve ever done.”
Spain’s twice world champion Fernando Alonso finished ninth in front of his home fans, after starting at the back of the grid, and AlphaTauri’s Yuki Tsunoda collected the final point.