Brought up in Kolkata, Sonnalli has finished her schooling from South Point High School and St Thomas Girls School. She started participating in beauty pageants at a very young age. She won several local competitions and pageants in Kolkata like the May Queen Ball. She also won the Pantene Teen Queen India and worldwide at the age of 15. In 2006, she won the crown of Miss India International and represented India at the Miss International pageant. She was among the top 12 finalists in the international platform.

Sonnalli has been a part of all the major fashion weeks like the Lakme Fashion Week and Wills India Fashion Week. She has walked the ramp for many prominent designers including Manish Malhotra, Sabyasachi Mukherjee, Rohit Bal, etc. Sonnalli has also been featured in several advertisements such as Raymond, Xylo (with Atul Kasbekar), ICICI, Castrol (with MS Dhoni), Thums Up (with Salman Khan and one with Mahesh Babu for south), Aristocrat (with Ajay Devgn).

She was also seen in many music videos including Atif Aslam (Tips), Prem, a Canadian singer (Times) and Dr. Zeus (Studio One).