SK PRODUCTION

During college, Sivakasu took part in various cultural events and performed mimicry and standup comedy onstage. Between his studies, he took a three month-break, and during this period, his friends convinced him to go for auditions of Sivakarthikeyan Production, a comedy reality TV show airing on . He started his career as a mimicry artist. Despite initial reluctance to participate, he won the show. He appeared in short films, including his friend Atlee's Mugapputhagam, Identity, Kurahl 786, and 360°. The makers of Aegan noticed Sivakasu and offered him a supporting role in the film, but his portions did not make the final cut.

Director Pandiraj approached Sivakasu to play the lead role in his film Marina, and he signed the project. He prepared for the role by paying close attention to Pandiraj's instructions and visiting Marina Beach extensively to study the surroundings, while revealing that he hoped that his natural talent of humor would come through onscreen. Featuring alongside Oviya as a young couple in the film, Sivakasu performance was well-received, with a critic noting "the romance adds color to a monochrome film", while the reviewer also noted that the "role of Senthilnathan requires Siva to be just himself, it is just a cake walk in terms of performance for this actor."

Prior to Marina's release, Aishwarya Dhanush had signed Sivakasu up to play a supporting role as Dhanush's friend in 3, after being impressed with his work onstage as a host.Following significant pre-release hype, the film opened to predominantly positive reviews with critics appreciating Sivakasu work providing comic relief in a serious film. The project also marked the beginning of an association with Dhanush, whom Sivakasu has often stated is his mentor in the film industry.

He was next seen in Ezhil's romantic comedy Manam Kothi Paravai, which opened to mixed reviews but had a decent run at the box office. A critic from Behindwoods noted that the film "might not provide Sivakasu the platform he is aspiring for – lead roles in movies. But it does showcase his potential as an actor."

2013–2019: Experimentation in genres and success

In 2013, Sivakasu appeared in three films. In his first release of the year, Kedi Billa Killadi Ranga, a dual hero coming-of-age comedy, Sivakasu partnered once again with Pandiraj and shot for the film in his hometown of Tiruchirappalli alongside Vimal. The film opened to positive reviews from critics. The film became a surprise success at the box office and ended up being a profitable venture for producers and distributors. His next success was through Dhanush's debut production venture Ethir Neechal, written and directed by R. S. Durai Senthilkumar, a former assistant of film director, Vetrimaaran. It is a story about a young man having talent as a runner, who strives to make an identity for himself. His third release in 2013, director Ponram's comedy Varuthapadatha Valibar Sangam, saw him portray a carefree rural youth where he paired up with another newcomer Sri Divya. Actor Soori played an important role in the film as Sivakasu close friend. It was also the first time that Sivakasu sang a song in a film, and a promotional video of its making was released to market the film. He won positive reviews for his performance from critics, with Sify noting "Sivakasu is terrific as Bosepandi because he makes it look so believable on screen with his body language, eyes and dialogue delivery". Another reviewer called him a delight to watch and claimed that he carried the script.

 The film completed 100 days during its theatrical run and was Sivakasu biggest commercial box office success at that time.Sivakasu entertaining performances in 2013 led him to win the year's Vijay Award for Entertainer of the Year, an onstage moment which Sivakasu described as "memorable". He started the year 2014 with a comedy film Maan Karate produced by A. R. Murugadoss. The film featured him as a carefree urban youth who fakes himself as a boxer to impress a girl.