Smith's most prominent international film til date is Gurinder Chadha's Viceroy's House (film) on India's partition in 1947 in which he plays the role of Muhammad Ali Jinnah.

Smith's father played the accordion, piano, violin and ukulele. A talented singer, Benjamin was granted a leave of absence to represent India as part of the classical a cappella Paranjoti Academy Choir for an extended tour of Europe in the 1960s. His father died suddenly when Smith was 11.

Smith also played key roles in numerous other productions such as Jawaharlal Nehru in Letters to a Daughter from Prison (directed by Vijaya Mehta and based on the 1984 publication of Nehru's letters), Jesus in Jesus Christ Superstar in Toronto, Canada, Vali and Vibishan in The Legend of Ram, Judge Brack in Hedda Gabler, and the solo performer in four monologues written and directed by Zubin Driver in Mumbai vs Mumbai.

Smith began his professional career as a Film Executive under Indian adman and actor, Alyque Padamsee, who was then CEO of the advertising agency Lintas. In 1988, Smith quit his corporate career to focus on stage acting. He trained in voice at the National Centre for the Performing Arts, Mumbai, and was guided by Dr. Ashok Ranade and Pratap Sharma.

By 1998, he also began working extensively with Lilette Dubey’s PrimeTime Theatre in productions such as On a Muggy Night in Mumbai, Zenkatha, Sammy, August: Osage County and "Gauhar" about the life of Gauhar Jaan. These plays toured both India and various cities in North America, UK and Europe, South-East Asia and Australia.

He is known for roles such as the Tibetan monk Lama Norbu in Paap (2003), directed by Pooja Bhatt and starring John Abraham, and as Tenzing in the film Frozen (2007), based in Leh and Ladakh. Among the many independent films he has worked in are Shobhayatra (2004), where he played Jawaharlal Nehru, The Memsahib (2006), Mumbai Salsa (2007), Chase (film) (2010), Lamhaa (2010), Impatient Vivek (2011), Tripura (2011), Ajita Suchitra Veera's Ballad of Rustom (2012), John Day (2013), and Dad... Hold My Hand! (2015). In 2017, Smith played the role of the Principal in the Marathi film Manjha (2017), directed by Jatin Wagle.

He's also been featured in many Bollywood films such as Ek Ajnabee (2005) with Arjun Rampal, Amitabh Bachchan and Parizaad Zorabian, Zid (2014) and Bombay Velvet (2015) directed by Anurag Kashyap with Anushka Sharma and Ranbir Kapoor, Kabir Khan's Phantom (2015) with Saif Ali Khan and Katrina Kaif. More recently, Smith appeared as the Manipuri militant leader in Disney's Jagga Jasoos (2017) with Ranbir Kapoor, directed by Anurag Basu and the emergency period action film Baadshaho (2017) with Ajay Devgn and Emraan Hashmi.

Smith founded Stagesmith Productions in 2006 with an aim to produce Indian English Theatre rooted in homegrown narratives. Its first production, titled Jazz, starred actor Bhargava Krishna, Saxophonist Rhys D’souza, and featured musical compositions by Merlin D’Souza. The play opened to a full-house at the Prithvi Theatre Festival in 2007, and won a 'Best Actor' award for Krishna at the Mahindra Excellence in Theatre Awards in 2008. The play has also been credited with laying the seed for Fernandes’ now-iconic book on the history of Jazz music in India and Goan Jazz Musicians in Bombay’s film industry – Taj Mahal Foxtrot: The Story of Bombay’s Jazz Age.

In 2007, Smith joined All Star Artists for the Broadway style musical production The Merchants of Bollywood – written and directed by Toby Gough and choreographed by Vaibhavi Merchant – in the lead role of the grandfather, Shantilal, and then later a double role that of "a splendidly-moustachioed narrator" and the sleazy Bollywood Director Tony Bakshi. There were 600 shows between 2007 and 2009 in the UK, Europe and Malaysia. and he reprised his role for a run in the UK and Lebanon in 2016. In 2017, Smith re-joined Director Toby Gough, Choreographer Shruti Merchant and team as the voice of Raj Pakoda in the light-hearted dance musical Taj Express.

Smith has featured in several international productions that have achieved both critical and commercial success – One Night with the King (2007) with Omar Sharif and Peter O’Toole, The Lunchbox (2013) with Irrfan Khan, The Best Exotic Marigold Hotel (2011) and The Second Best Exotic Marigold Hotel (2015) with Judi Dench, Maggie Smith, Dev Patel, Bill Nighy and directed by John Madden and Brahman Naman (2016), a Netflix original release sex comedy, directed by Qaushiq Mukherjee, written by Naman Ramachandran with Shashank Arora.

In 2015, StageSmith revived Jazz, and renamed it Bombay Jazz with Smith himself playing the mentor – an amalgamation of jazz Musicians Sebastian D’Souza, Chris Perry, Micky Correa, Chic Chocolate (also known as Louis Armstrong of India) Frank Fernand and Anthony Gonsalves – with Saxophonist Rhys Sebastian D’Souza playing the mentee.

In 2016, StageSmith created a new production "Poetrification" dubbed as "Your moment to get smashed by some poetry & music." In the performance, Denzil Smith, Danish Husain, and a musician craft a performance of poetry, music, and banter. Denzil performs pieces by English poets from the sub-continent - Dom Moraes, AK Ramanujan, Jeet Thayil, Arundhathi Subramaniam - while Danish recites Urdu poets - Ghalib, Noon Meem Rashed, Faiz, Zehra Nigah, Afzal Ahmed Syed. Through this, Adil Manuel or Beven Fonesca weave words with music.

Smith’s love of music – Jazz in particular – has translated into being a regular host of jazz festivals, music and other events. He regularly hosts Jazz tribute concerts on International Jazz Day at the National Centre for the Performing Arts and Blue Frog in Mumbai. He also co-hosted the META awards in 2014 with Perizaad Zorabian.