Lastra completed vocational training classes in electronics before starting his scenic career, although he claims he was a "poor student." After being inspired to pursue a musical career after seeing David Bustamante's performances in Operación Triunfo, he began his studies in this field in 2003, taking voice technique and musical performance lessons from Victoria Manso and Patricia Ferro.
In 2004, he began his career in musical theatre. Since then, he has been in Broadway Millennium, Hoy no me puedo levantar, En tu fiesta me colé, Flashdance, 40, the musical, Billy Elliot, and El discurso del rey, among other theatrical productions. His first major television role was as 'Pipo' in the Corta-T series, which aired on Cuatro.