Upvote:3
Citizens of The United States Of America are free to travel throughout a schengen zone for 90 days or less without restriction.
Upvote:9
If you're going to Lisbon, Frankfurt is where you're going to go through the Schengen border controls anyway. The flight from Frankfurt to Lisbon is within Schengen and doesn't involve passport checks (except what the airline does at the gate to make sure that the person who flies is the same guy they sold a ticket to).
Once you have arrived at Frankfurt and cleared the border checks you're free to leave the airport.