Entering the industry in 1999 as a screenwriter on director Gopal B's Samarasimha Reddy, M. Ratnam has established himself as a writer of story, dialogue and screenplay. Over the years he has worked on movies like Simhadri (2003), Sye (2004), Chatrapathi (2005), Vikramarkudu (2006), Rakhi (2006), Raju Bhai (2007), Lakshyam (2007), Yamadonga (2007), Souryam (2008), Magadheera (2009), Rama Rama Krishna Krishna (2010), Legend (2014), Sarrainodu (2016) and Jaya Janaki Nayaka (2017). In 2018 Ratnam served as the writer on director KS Ravikumar's action-drama, Jai Simha. The cast has Nandamuri Balakrishna, Nayanthara and Haripriya in pivotal roles. He is associated with the 2018 Telugu action-romance Juvva directed by Triekoti Peta.