Garrett was born on April 10, 1955, in the village of Thorne, South Yorkshire, England. She graduated from London's Royal Academy of Music in 1979 and immediately went on to win the prestigious Kathleen Ferrier Prize. She debuted at the 1980 Wexford Festival in Mozart's singspiel Zaide. After further successful appearances at Opera North and with the Glyndebourne Touring Opera, she joined the English National Opera (1984) and became the company's principal soprano, distinguishing herself over the next two decades in a variety of Mozart roles, including Zerlina (Don Giovanni) and Despina (Così fan tutte), as well as in a mixture of others: Dalinda from Handel's Ariodante and her widely acclaimed Rosina from the 1998 production of Rossini's The Barber of Seville.
As her fame grew, Garrett's television and radio appearances became more frequent. Along with the aforementioned BBC shows, she made appearances on the popular genealogy program Who Do You Think You Are in 2004, hosted the ITV production of the Classical BRIT Awards (2005), and appeared regularly as a judge on the Comic Relief Does Fame Academy (2007). Among Garrett's later recordings is her 2007 Kultur Video release Music From the Movies.