Sinclair has mentioned Kate Bush and Alanis Morissette as her most important musical influences, describing them as emotional artists "who write honestly about their opinions and experiences." She also stated she considered Gwen Stefani, who is a musical and stylistic inspiration, as her "hero".
She began pursuing a music career and had a significant breakthrough as "Candy Rapper" in 2003, when she won a BBC Radio 1 MC Battle in Leeds. Kills released her debut album Perfectionist in Germany in April 2011 following the release of her first single, "Mirrors", which hit the German Top 10. Her second studio album Trouble, was officially released on 3 September 2013 as a digital download in the United States and Canada. In early 2016, Sinclair started a band, Cruel Youth, with (her husband) Moon. Their first song, "Mr. Watson", was released on Soundcloud in February, followed by "Diamond Days" in April, which was also released on iTunes. Their third single, "Hatefuck", was released in early September, along with the release of their debut EP, +30mg, on 16 September 2016.