Lassalle has served as mayor of the commune of Lourdios-Ichère since 21 March 1977, and has sat on the conseil général of the département of Pyrénées-Atlantiques since 22 March 1982, serving as vice-president since 1 January 1991. He was elected to the National Assembly in the 2002 legislative election, and represents the 4th constituency of Pyrénées-Atlantiques. Since 2002 Lassalle has led the World Mountain People Association, an international network of mountain-dwellers active in more than 70 countries. He also leads a Haut-Béarn cultural association.
On 3 June 2003 Lassalle stood up in the National Assembly during questions to Minister of the Interior Nicolas Sarkozy and sang the Occitan anthem Se Canto in protest at an announcement by Sarkozy concerning the housing of 23 gendarmes tasked with guarding the Somport tunnel, which links France with Spain through the Pyrenees. The village closest to the French end of the tunnel is Urdos, but it was announced that the gendarmes would be housed in the nearby town of Oloron-Sainte-Marie, on the grounds that their wives would become bored in Urdos. As he explained in an interview with France 3 later that day, Lassalle took exception to what he saw as a slur on the Pyrenean village and decided to interrupt the minister with his song. The protest was met by laughter from other deputies, disapproval from the president of the Assembly, and bemusement from Sarkozy.
In 2013, Lassalle walked around France for eigtht months from April to December to meet people. He was afterwards quoted “Everywhere I went I witnessed a crisis in the standard of living, a loss of identity and the loss of a sense of a common destiny,”. He found the situation equally bad in the cities and the countryside. Scepticism about globalisation, distrust of politicians and latent racism were common among people he spoke to, he said.
Lassalle undertook a 39-day hunger strike in 2006, in protest at a threat to jobs in his constituency. Japanese firm Toyal, which owned a paint factory near Accous in the Vallée d'Aspe and employed 150 of Lassalle's constituents, announced plans to open a factory 60 km away. The firm claimed that this relocation of its investment would not result in the closure of the Accous site, but Lassalle's concerns were not assuaged and he embarked on a hunger strike on 7 March.
The strike ended on 14 April, when Toyal offered an assurance to Minister of the Interior Nicolas Sarkozy that the factory would not close, and that it would continue investment in the area before expanding elsewhere. Earlier the same day he had been admitted to hospital in Garches, in the western suburbs of Paris, prompting intervention by President Jacques Chirac, Prime Minister Dominique de Villepin and Sarkozy. Lassalle had lost 21 kg over the course of the strike.
- His son Thibault Lassalle is a very promising rugby union footballer. He was a member of the French U19 team taking part in the 2006 U19 World Championships in Dubai in which France reached the semi-final.
- "Jean Lassalle". Assemblée nationale. Retrieved 14 April 2006.
- "Singing MP baffles Paris parliament". BBC News. 3 June 2003.
- MP finds France ‘without hope’ after walking tour of country France24. 13 December 2013. Retrieved 19 December 2013
- "French MP hunger strike wins deal". BBC News. 14 April 2006.
- Official website (in French)
- Official website (in English; less information)
- World Mountain People Association