He is best known as the founding member of the rock band Focus as its primary vocalist, keyboardist, and flautist.
Born and raised in Amsterdam among a musical family, van Leer took up the piano and flute as a child and pursued them at university and music academies.
In November 2021, Thijs was diagnosed with Covid-19 after being hospitalized during a Focus tour in the UK.
He is a Dutch musician, singer, songwriter, composer and producer.
On 13 September 2008, van Leer was made a Knight of the Order of Orange-Nassau for "special services to music.".