The London side made a great start to the game as Timo Werner missed two huge chances in the first 15 minutes.
Chelsea center-back Thiago Silva, the oldest player on the pitch, was subbed off due to injury and replaced by Andreas Christensen in the 39th minute.
The first half ended 1-0 in front of 16,500 fans at the Estadio do Dragao stadium in Portugal's Porto.
In the 73rd minute, Chelsea could double their lead, but Pulisic missed another massive chance when he was one-on-one with Ederson.
Manchester City failed to score in the remaining of the game and the final ended 1-0, which means Chelsea claimed their second Champions League trophy.
They won their first title in 2012 by beating Bayern Munich on penalties.