Sandhya was an ordinary school girl when she starred in the critically acclaimed Tamil love story Kaadhal. Initially, this role was offered to actress Gopika, however, due to date problems, this role went on to Sandhya. She planned to do the Visual Communications course after her plus two. However, due to her rising fame, especially after the blockbuster film Dishyum, her second Tamil film, she had to discontinue her studies before even passing her 10th standard.
Kaadhal, for which she won the Filmfare Best Tamil Actress Award of 2004, she took up a Malayalam film titled Alice in Wonderland which she played as the sister of actor Jayaram. Then came the musical romance Dishyum with Jeeva. She was also a part of Mani Ratnam's stage show, Netru, Indru, Naalai. In her third Tamil film, Vallavan, she had only a small role, and the film had only modest box office success. She also acted in Koodal Nagar with Bharath. She will be debuting in playback singing for her new Tamil film Manjal Veiyil.
She was approached for the voice-over for Shriya Saran's character in Sivaji, however, S. Shankar later preferred actress Kanika Subramaniam's voice.