The film will also feature producer Sajid Qureshi's daughter Amrin.
"Rajkumar had several rounds of auditions before deciding on them and put both of them through some acting workshops," Qureshi said in a statement.
The producer also revealed the film would be on the lines of Shahid Kapoor and Ileana D'cruz's "Phata Poster Nikhla Hero", and "Ajab Prem Ki Ghazab Kahani" featuring Ranbir Kapoor and Katrina Kaif.
Mithun was in the Us when Namashi was signed for the film, Qureshi said.
"But on his return, he met us and discussed the script. Any father would be pleased to have his child launched by Rajkumar Santoshi. It means he is in safe hands," he said.
The film will be shot in Bengaluru and Mumbai over the next 65 days.