It is not possible to go to the Johto region in "Pok?mon Leaf Green." However, it is possible to capture a large variety of Pok?mon from the Johto region after defeating the Elite Four by going to several islands called the Sevii Islands.
The first Sevii Island can be reached after the player has defeated Cinnabar Island's gym leader, Blaine. After the player has defeated the Elite Four he will be able to access every Sevii Island. Pok?mon that come from the Johto region can be found on the Sevii Islands, as well as one of the three Legendary Dog Pok?mon from the Johto region. The dog Pok?mon will be either Suicune, Entei or Raikou. The game chooses the dog that has the type advantage over the Pok?mon that the player chose at the start of the game.
The player can do a lot more than simply catch Johto Pok?mon on the Sevii Islands after defeating the Elite Four. He can also go to Mt. Ember and retrieve the sapphire from Team Rocket's goons or get the ruby from a Team Rocket warehouse. The entrance to Cerulean Cave is also unblocked so the player will be able to go there and capture the legendary Pok?mon Mewtwo.