Forbes L’Estrange began working with the a cappella group The Swingles as soprano and musical director. From 1998 until 2000, she led the group in concerts in North America, South America, Asia, Europe and Australasia. She appeared in three contemporary operas at Teatro alla Scala, Milan, and at Théâtre du Châtelet in Paris. Forbes L'Estrange worked with musicians including the late Ward Swingle and Luciano Berio, whose orchestral work Sinfonia she has performed with orchestras worldwide.

As a soloist, she specialises in contemporary crossover music including Mass in Blue by Will Todd (recorded in 2019 for Convivium Records) and Sacred Concert (Ellington). She sings jazz with "L’Estranges in the Night", the duo she co-founded with Alexander L'Estrange. As an ensemble singer, she sings with London-based, professional vocal ensembles including London Voices - directed by Ben Parry (musician), Tonus Peregrinus (vocal ensemble) - directed by Antony Pitts and Synergy Vocals - directed by Micaela Haslam, performing Minimal music by Steve Reich and John Adams (composer). She has recorded over twenty CDs with Tenebrae (choir), directed by Nigel Short (singer). She records film soundtracks at London's Abbey Road Studios. Her voice features on the soundtrack to the Wes Anderson 2021 film The French Dispatch in a duet with Jarvis Cocker.

Forbes L’Estrange is "one of the Royal School of Church Music’s best-selling composers". She was commissioned to compose an anthem to mark the coronation of King Charles III in May 2023. The mountains shall bring peace, a setting of Psalm 72 and Psalm 149, was performed by choirs throughout the world. Forbes L’Estrange founded, and for five years directed, AQUILA, the all-female vocal group of St John’s College, Cambridge. She has been a guest conductor for the National Youth Girls Choir and Choir of the Earth. In 2018, she organized and conducted the first all-female recording session at Abbey Road Studios, recording her song Twenty-first-century Woman. She was a judge on the Sky 1 series Sing: Ultimate A Cappella.