Dalot put Portugal ahead in the seventh minute when Bruno Fernandes played a clever chipped pass into his path and the defender went past Borna Sosa before firing his shot through the legs of the keeper.
Ronaldo, who failed to score in Portugal's five Euro 2024 games, reached his milestone when he got on the end of a perfect cross from Nuno Mendes and the unmarked 39-year-old tapped in from close range to double the lead in the 34th minute.
Croatia pulled one back four minutes from the break when Dalot deflected a sliding effort from Sosa into his own net but a scoreless second half gave Portugal a winning start to their campaign.
In Thursday's other League A Group 1 game, Poland beat Scotland 3-2. Portugal host Scotland and Croatia entertain Poland on Sunday.
