Khemu was born to actors Ravi and Jyoti Kemmu in a Kashmiri Pandit family in Srinagar, Jammu and Kashmir. He attended Niranjanlal Dalmia High School in Mira Road, Mumbai and pursued higher education at Amity University.
His grandfather, Moti Lal Kemmu (original spelling of last name), is a Kashmiri playwright and a recipient of several awards by the government of Jammu and Kashmir: Rashtra Bhasa Prachar Samiti, Sahitya Akademi award in 1982 for his contribution to Kashmiri literature as a playwright and Padma Shri.
Khemu was in a live-in relationship with actress Soha Ali Khan since May 2013. The couple got engaged in July 2014 and got married in a private ceremony in Mumbai on 25 January 2015 in the presence of selected members of Soha's family.
He made his debut as a child actor appearing in the Doordarshan TV series Gul Gulshan Gulfaam (1987), directed by Ved Rahi. He made his film debut with Mahesh Bhatt's movie Sir (1993). He went on to star as a child artist in movies including Raja Hindustani, Zakhm, Bhai and Hum Hain Rahi Pyar Ke.
He played the male lead in the 2005 film Kalyug, which was directed by Mohit Suri. Kalyug was based on the pornography industry. In 2007, his first release was Madhur Bhandarkar's Traffic Signal, where he played a street-smart money lender who lends money to poor families, but always gets it back with interest.
His second release of 2007 was Dhol in which he again played lead. In 2008, in his only release, Superstar, he featured in a double role. In 2009, he starred in Dhoondte Reh Jaaoge and Jai Veeru. In 2009, he appeared in the comic-thriller 99. In 2010, he appeared in Golmaal 3 in which he played a supporting role as Laxman.
In 2012, he appeared in Mukesh Bhatt's Blood Money, in which played the lead role as Kunal Kadam, an honest and hard worker who is unknowingly taken into the underworld by his senior boss. He co-starred in Go Goa Gone, a zombie film, which was released on 10 May 2013.
In 2015, after over a two-year break, Khemu returned to the screen with the thriller Bhaag Johnny about a man who gets to live two lives by the help of a genie, portrayed by director Vikram Bhatt. Though the film began shooting in 2013, it released on 25 September 2015 to mixed reviews and low box office collections. His second release of the same year was the comedy film Guddu Ki Gun, in which Kunal appears in the title role.