After auditioning for Sir John Eliot Gardiner she joined the Monteverdi Choir, singing in many of the concerts of the Bach Cantata Pilgrimage which the choir completed in the year 2000. In 2001 she moved to pursue postgraduate vocal studies at the Royal College of Music in London.
She went on to sing with The Sixteen, Polyphony, Cambridge Singers and the Gabrieli Consort, as well as pursuing a solo career. She is the first singer to record Bach's Alles mit Gott, a birthday ode written in 1713 and discovered in 2005. She first received great acclaim for her 'Pie Jesu' on Naxos' award-winning recording of Rutter's Requiem.
In 2016, Manahan Thomas was the soprano soloist in the première of Karl Jenkins' choral work, Cantata Memoria: For the children, at the Wales Millennium Centre.[7] She also features on the première recording of the work.
On 19 May 2018 she sang Eternal source of light divine at the wedding of Prince Harry and Meghan Markle, accompanied by the event's orchestra and with David Blackadder playing the trumpet obbligato.[8]
A Christmas Festival (2008), recorded with the Cambridge Singers, Farnham Youth Choir and the Royal Philharmonic Orchestra, conducted by John Rutter[10]
Song of Songs (2009), composed by Patrick Hawes, featuring Hawes's choir Conventus and the English Chamber Orchestra. [11]
^'Appendix V. Candidates who Took the Anglo-Saxon, Norse and Celtic Tripos between 1900 and 1999', in H. M. Chadwick and the Study of Anglo-Saxon, Norse and Celtic in Cambridge, ed. by Michael Lapidge [=Cambrian Medieval Celtic Studies, 69–70] (Aberystwyth: Department of Welsh, Aberystwyth University, 2015), pp. 257–66 (p. 265). ISBN978-0-9557182-9-8.