Lemieux was born in Dolbeau, Quebec, province, Canada, in 1975. She began serious vocal studies in 1994 at the Conservatoire de musique de Chicoutimi, where her most important teacher was Rosaire Simard. She followed with studies at the Conservatoire de musique de Montreal, where she worked with Marie Daveluy.
Lemieux's victory in June 2000 at the Queen Elizabeth Competition was preceded by triumphs at two important Canadian competitions: in March at the Orchestre symphonique de Trois-Rivières Competition and in May at the Jeunesses Musicales du Canada National Competition.
Lemieux's first solo recording was issued in November 2000 by Cypres, a CD containing Berlioz' Les nuits d'ete and lieder by Mahler and Wagner. Her operatic debut as Cornelia in Handel's Giulio Cesare took place at the Canadian Opera in April 2002. Critics and public alike responded with enthusiasm. 2005 was a pivotal year for Lemieux: she sang the title role in Vivaldi's Orlando furioso on a highly praised three-disc Naïve set; later that year she debuted to critical acclaim at the Staatsoper Berlin in Monteverdi's Il ritorno d'Ulisse in patria. Lemieux's U.S. recital debut took place in November 2007 in Overland Park, KS, with pianist Daniel Blumenthal accompanying, in works by Brahms, Schumann, Debussy, Mozart, and Reynaldo Hahn. Her U.S. concert debut came the following month with the Los Angeles Philharmonic in a performance of J.S. Bach's Magnificat. She continues to appear both as a choral soloist and in operas throughout Europe and North America. Among Lemieux's later recordings is a 2007 CD of Brahms lieder, issued on the Analekta label, and 2010's Ne me refuse pas, a collection of French arias.