Tanna was born and brought up in a Gujarati family. She lives with her mother, and is very close to her, as her father died in October 2012.

Tanna appeared in Balaji Telefilms' soap opera Kyunki Saas Bhi Kabhi Bahu Thi (July 2000 - November 2008), where she played the character Indu. She appeared in Balaji Telefilms' drama production, Kahi To Milenge (November 2002 - 2003).

In March 2016, Karishma made a guest appearance in the popular stunt reality show Fear Factor: Khatron Ke Khiladi in its seventh season. Tanna appeared as a celebrity guest in the popular comedy show Comedy Nights Bachao, which aired on Colors TV one with Upen in February and later in the same year in August 2016 for promoting Jhalak Dikhhla Jaa. She again made a small cameo appearance in Comedy Nights Bachao along with Helly Shah for one episode.Karishma participated in the celebrity dance reality show Jhalak Dikhhla Jaa in 2016.Tanna played a naagin in Life OK's show Naagarjuna – Ek Yoddha. In January 2017, Tanna was judging BIG Magic's reality show Big Memsaab along with Sambhavna Seth and Pritam Singh in 2017.