Carris Jones, a graduate of the University of Cambridge and the Royal Academy of Music, will become the new Alto Vicar Choral in September.
Carris, who is expecting her first child at the beginning of May, said: "I am thrilled to be appointed as a Vicar Choral at St Paul's Cathedral, and feel incredibly lucky to join a group of such friendly and talented musicians.
"I have long admired the cathedral as an organisation that manages to combine a venerable historical tradition with a vital place in 21st century London life, and am honoured to become a part of it."
A member of the Choir of the London Oratory, Carris has undertaken solo roles in oratorio and opera.
She has also performed with groups including Stile Antico, the Gabrieli Consort and Tenebrae.
St Paul's Director of Music, Andrew Carwood, said: "We are delighted that Carris is joining the Choir. The final round of auditions was of a very high standard and her appointment was a unanimous decision."
The Cathedral Choir currently comprises of 26 boy treble Choristers, eight probationers and twelve Vicars Choral - the professional adult singers who take the alto, tenor and bass parts.