Before the release of Boys, Siddharth signed on to feature in his mentor Mani Ratnam's political drama film, Aaytha Ezhuthu (2004) where he played the student Arjun Balakrishnan, a role which he described was similar to his real life persona.
He noted that he knew he was headed for a career in films since childhood, showing particular fascination for writing and directing, and thus only signed up for business school as a "safety cushion" as insisted upon by his father.He worked on the film throughout 2001, also making an uncredited appearance as a passenger on a bus.He has been featured in advertising because of his "pan-Indian" appeal.After completing his business management studies, Siddharth chose to work in film making and assisted Mani Ratnam, before going on to make an acting debut in S. The success of the film provided an opportunity to feature in Mani Ratnam's multi-starrer Aayutha Ezhuthu (2004), before he branched off to receive critical and commercial acclaim in Telugu and Hindi cinema through Nuvvostanante Nenoddantana (2005), Rang De Basanti and Bommarillu (2006).He was further attracted to the offer through the opportunity of working with Aamir Khan as well as by the film's ensemble cast of Madhavan, Sharman Joshi, Kunal Kapoor and Soha Ali Khan.
The drama film tells the story of an incident of government corruption radicalising a group of friends from being carefree to passion-driven individuals.
The actor then had a prolific year in 2014, winning critical acclaim and box office success for his two ventures; Jigarthanda, where he portrayed an aspiring film maker, and Kaaviya Thalaivan, in which he played an actor from the 1920s Madras theater scene. Jain Institute of Management and Research, Mumbai, while also ultimately winning a speaking skills competition which earned him the CNBC Manager of the year award in 1999.
Siddharth participated extensively in extra-curricular activities during college, serving as the president of the college's debating society and attending the World Debating Championships. Siddharth's first tryst with media came through dubbing for the Banish mosquito repellent ad in eight different languages in 1988, as instructed by his father's close friend, ad director Jayendra.
In the late 2000s, he established himself as a bankable lead actor in Telugu films and subsequently chose to become more selective with his projects, while also winning acclaim for his work as a carrom player in Striker (2010) and as a blind warrior in the fantasy film Anaganaga O Dheerudu (2011).
In 2011, he returned to Tamil films after a sabbatical and produced Balaji Mohan's commercially successful romantic comedy Kadhalil Sodhappuvadhu Yeppadi (2012).
He also was insistent that he dubbed for himself and learned the Telugu language during the making of the film.