Kelly was born and raised in New York City. His Jamaican-born mother introduced him to a variety of music styles, including jazz, reggae, blues and R&B. Growing up, he studied piano and flute at the Third Street Music School Settlment and sung with the New York Boys Choir. Kelly attended Grace Church School, Riverdale Country School, and then Berklee College of Music in Boston, Massachusetts, graduating with a degree in music business/management in 2002.
After graduating from Berklee, Kelly worked as a session singer on the East Coast, before starting to write his own songs. He placed his first song in 2002, when "You're Taking It" was included on a CD compilation for Japanese clothing line A Bathing Ape. He wrote "Daddy's Little Girl" for Frankie J, which was released as a single in 2006, and led to a publishing contract with Warner/Chappell Music. That year, Kelly was introduced to producer RedOne, and began working with him and Lady Gaga, who was at the time working on her first album. This led to an introduction to Akon. Kelly and Akon co-wrote "Hold My Hand" with Whitney Houston in mind, but Akon liked the song so much that he decided to record it as a duet with Michael Jackson. It was one of Jackson's last recordings, and appeared on his 2010 posthumous album Michael.
Nicknamed the Studio Beast, Kelly has written numerous hits, including the Britney Spears song "Circus", which he wrote with Dr. Luke and Benny Blanco and reached #3 on the Billboard Hot 100, and #1 on the Billboard Pop Songs and Billboard Hot Dance Club Play charts; Kelly Clarkson's "My Life Would Suck Without You", which he co-wrote with Max Martin and Dr. Luke and hit #1 on the Billboard Hot 100; and the Miley Cyrus single "Party in the U.S.A.", written with Dr. Luke and Jessie J, which spent 28 weeks on the Billboard Hot 100 and peaked at #2. He has been nominated for four Grammy Awards: the 2011 Grammy Award for Best R&B Song for Fantasia's "Bittersweet"; two 2012 Grammy Awards, for Song of the Year for Bruno Mars' "Grenade" and Best R&B Song for Ledisi's "Pieces of Me"; and the 2013 Grammy Award for Best R&B Song for Tamia's "Beautiful Surprise". He was named to The Hollywood Reporter's 2013 list of Music's Top 35 Hitmakers.
Kelly has written songs for Whitney Houston, R. Kelly, Miley Cyrus, Olly Murs, Jessie J, Britney Spears, Leona Lewis, John Legend, Faith Evans, Joe Jonas, Jason Derulo, Backstreet Boys, Adam Lambert, Martina McBride, Christina Aguilera, One Direction, The Wanted, and the K-pop girl group Wonder Girls. He co-executive produced Karmin's 2012 EP Hello and Tamia's 2012 album Beautiful Surprise, and co-produced Masha's debut rock EP Stupid, Stupid Dreams in 2013.
In 2014, Kelly and fellow songwriter and producer Chuck Harmony formed the R&B duo Louis York. Their debut EP, Masterpiece Theater – Act I, was released in 2015. The first single, "Clair Huxtable", is an ode to Phylicia Rashad's character on The Cosby Show. The EP was the first release from their music collective Weirdo Workshop, with distribution from Sony's RED Music.
Kelly served as a vocal producer for the US version of The X Factor in 2011 and 2012. In August 2013, it was announced that he and Jenna Hally Rubenstein were partnering to executive produce and develop a reality television show to find undiscovered music moguls.