Aggarwal made her acting debut with the 2004 Bollywood film Kyun! Ho Gaya Na... and had her first Telugu film release in 2007, Lakshmi Kalyanam. In the same year, she starred in the box office hit Chandamama, which earned her recognition. The 2009 historical fiction Telugu film Magadheera marked a turning point in her career, earning her critical acclaim. It ranks among the highest-grossing Telugu films of all time and fetched her Best Actress nominations at several award ceremonies including Filmfare.
She subsequently starred in Telugu films such as Darling (2010), Brindavanam (2010), Mr. Perfect (2011), Businessman (2012), Naayak (2013), Baadshah (2013), Govindudu Andarivadele (2014), Temper (2015) and Khaidi No. 150 (2017). Kajal also played the female lead in the high-profile Tamil projects Naan Mahaan Alla (2010), Maattrraan (2012), Thuppakki (2012), Jilla (2014),Vivegam (2017) and Mersal (2017). She made a comeback to Bollywood with Singham (2011), which was a hit, while another film Special 26 (2013) was also a box office success.