From 1967 to 1969, van Leer was backing vocalist and musician in a theatre cabaret act headed by Ramses Shaffy; recorded singles as a solo artist; and produced, arranged, and conducted music for Bojoura.

He formed Trio Thijs van Leer, a three-piece rock band which evolved into Focus in late 1969 following the addition of guitarist Jan Akkerman.

Focus achieved international success following the release of Moving Waves (1971) and its lead single, "Hocus Pocus", which features van Leer's yodelling and whistling.

[1] His father Ed, a Jewish refugee from the Netherlands, was a classically trained flautist and businessman;[2][3] his mother Mary was a singer and involved in the Sufi Movement.

[4][3] Van Leer began playing the piano at age three, taught by his mother,[2] and later received lessons from famed pianists Maria Stroo (later Marja Bon) and Gerard Hengeveld.

[8] He joined the Raoul Angenot Quintet, and at eighteen, won an award at inter-school contests for playing Andante in C Major by Mozart on flute, singing an original love poem to "Stella by Starlight" by Victor Young, a drama improvisation featuring his own poetry, and an original piano composition titled "Nooit Zal Ik Vergeten (De Nachten Samen met Jou)" ("Never Shall I Forget (The Nights Together with You)".

[10] After graduating, van Leer did not feel confident enough to attend a music school, so he studied art history at Amsterdam University which he found "very dry".

This landed him an invitation from drummer Hans Cleuver in September 1968 to play the flute with bassist Martijn Dresden and himself on Jazz and Poetry, a program on the Catholic radio station KRO.

[15] Van Leer's final show with Shaffy was Sunset Sunkiss, which received an album release in 1969 on the Philips label with Cleuver and Dresden in the band.

[16] This was followed by several performances from the group at some large venues, including shows at the Holland Festival backed by the Metropole Orchestra, the Carré Theatre, and the RAI Centre.

After a line-up change, the band released Moving Waves (1971) that included their international hit single "Hocus Pocus" which featured van Leer's distinct vocals,[20] yodelling, whistling, and scat singing.

His influence from the Sufi movement through his mother is displayed in "Moving Waves", a piano and vocal composition he wrote when he was sixteen and quotes a teaching from its creator, Inayat Khan.

[17][21][22] The album closes with "Eruption", a 22-minute track that is "based on a musical idea" by van Leer who gained inspiration from the operas Orfeo ed Euridice by Christoph Willibald Gluck and L'Orfeo by Claudio Monteverdi, combining rock, jazz and classical influences.

It features a selection of classical pieces by Bach and Gabriel Faure, Focus songs, and original compositions arranged for flute and orchestra by van Otterloo, and soprano vocals by Letty de Jong.

[33] In 1981, van Leer formed the multi-national band Pedal Point with musicians Tato Gomez and Mario Argandoña [es] from Chile and Paul Shigihara from Japan.

Van Leer was one of the artists who recorded the song Shalom from Holland (written by Simon Hammelburg and Ron Klipstein) as a token of solidarity to the Israeli people, threatened by missiles from Iraq, during the first Gulf War in 1991.