Madrid.
Leon has an airport but service to it is very limited; there are not flights to Paris or London.
AENA manages Spains airports. Find airline and destination information for each airport.
http://www.aena.es/csee/Satellite/HomeAena
Renfe provides frequent service to Leon from anywhere Spain. Trains leave Madrid from Chamartin. Alvia trains make the trip in about 3 hours.
http://www.renfe.com/index.html
ALSA provides frequent service from Madrid Estacion del Sur and Madrid Intercambiador Moncloa. There are a couple buses that stop at T4 Madrid airport.
http://www.alsa.es/en/
2007
Madrid Airport to train station Chamartin
a. Chamartin is reached by Metro. Take line 8 from airport T1-T2-T3 or T4 to Nuevos Ministerios; and, line 10 to Chamartin.
b. Chamartin is reached by Renfe Cercanias direct from Madrid Airport T4.
2012
Madrid Airport to Estacion Sur de Autobuses de Madrid
Estacion Sur de Autobuses de Madrid is located near the Méndez Álvaro Metro Station. Take line 8 from Barajas airport T1-T2-T3 or T4 to Colombia; line 9 to Avenida de America; and, line 6 to Méndez Álvaro. Exit the Metro to find the bus station.
2013
Madrid Airport to bus station Moncloa
Madrid Moncloa is co-located with the Metro. Take line 8 from Barajas airport T1-T2-T3 or T4 to Nuevo Ministerios; and, line 6 to Moncloa.
1803
Leon Trains
Leon Renfe Station
http://www.adif.es/en_US/infraestructuras/estaciones/15100/informacion_000037.shtml
1804
Leon Bus Services
Leon Bus Station
http://www.leon-virtual.es/autobuses/2-7-4-7.htm