Mercado was born in Bridgeport, Connecticut on January 2, 1987. Her mother Zelda, a former Motown backup singer, is African-American and her father, José, is of Cuban descent. She grew up in Sarasota, Florida, attending Daughtrey Elementary School in Manatee County. Dreaming to become a gospel singer one day, she signed a contract at age nine with the Take Stock in Children initiative, a scholarship program for "low-income families who show potential for college but are unlikely to be able to afford a higher education." She graduated from Booker High School in 2005, where she was in the Theater VPA (Visual and Performing Arts) department.

At the 2005 Florida Theatre Conference and the 2005 South Eastern Theater Conference Syesha won "Best Supporting Actress" for her role as the Sour Kangaroo in Seussical the Musical. Mercado also appeared as a contestant on the failed Abctalent show, The One: Making a Music Star in summer 2006. She currently attends Florida International University in Miami, Florida.

She auditioned for American Idol Season 7 in Miami, Florida singing "Think" by Aretha Franklinwhere she made it to Hollywood with all three judges saying 'yes'. When she arrived in Hollywood she had lost her voice and insisted on going on vocal rest. She was seen communicating through pen and paper throughout the week. She sang "Chain of Fools" and was accepted into the top 24. She was the only female in the top 3 and was voted off on May 14th allowing fellow contestants David Archuletaand David Cooka chance to compete in the finale.

Despite fairly good reviews from the judges, Mercado had been in the bottom two or three five times without being voted off. Some commentators, including American Idol judge Simon Cowell believed her style was more suited for Broadway than pop music. According to her AmericanIdol.com profile, Mercado's musical influences are Whitney Houston, Etta James, Alicia Keys, Zap Mama, Lauryn Hill, and Aretha Franklin

After her elimination Mercado appeared on Live with Regis and Kelly. She will join the American Idols LIVE! Tour 2008 from July 1, 2008 to September 13, 2008.