Born in Paris, Lejet was the youngest of three children in a family with an engineer father.

[1] Lejet studied with Marcel Beaufils (aesthetics), Henri Challan (harmony), Marcel Bitsch (counterpoint and fugue), Jean Rivier and Andre Jolivet (composition) at the Paris Conservatory.

She was in residence at the Casa de Velázquez in Madrid from 1968 to 1970.

Lejet taught harmony at the Sorbonne from 1970 to 1972, and subsequently became professor at the National Conservatory of Music and Dance in Paris.

In 2004, Lejet was appointed professor of composition at the École Normale de Musique de Paris Alfred Cortot.