Dev (Devesh) Patel was born in Brent, Greater London, England, to Hindu Gujarati Indian parents, both of whom were born in Nairobi, Kenya. His mother Anita Patel is a carer and his father Raj Patel an IT consultant. Dev Patel, who grew up in Harrow, attended Longfield Primary School and later went to Whitmore High School, receiving an A* in GCSE Drama, for his "self-penned portrayal of a child in the Beslan school siege". His drama teacher Niamh Wright has stated: "Dev was a gifted student who quickly impressed me with his innate ability to communicate a wide variety of characters imaginatively and creatively. [...] He was awarded full marks for his GCSE performance to a live audience and the visiting examiner was moved to tears by his honest portrayal of a child in the Beslan school hostage crisis in Russia." He eventually completed his AS Levels in 2007, while he was working in Skins.

Patel started training at the Rayners Lane Academy of Taekwon-do in 2000. He competed regularly in both national and international championships, including the 2004 AIMAA (Action International Martial Arts Association) World Championships in Dublin, where he won a bronze medal. The World Championships took place in October 2004, when he was a red belt competing in the junior division against other red and black belts. He made it to the semi-finals, where he lost to a black belt, and ended up winning a bronze medal. He later gained a 1st dan (rank) black belt on 11 March 2006.

Patel began his acting career in late 2006, when he auditioned for the E4 TV teen drama series Skins. After two auditions, he was cast in the role of Anwar Kharral, a British Pakistani Muslim teenager. According to fellow actors Mike Bailey (who played Sid Jenkins) and Hannah Murray (who played CassieAinsworth), the characterization of Anwar was partly based on the personality of Patel and the role was written specifically for him after he was cast in Skins. The first series of the show aired in early 2007 and went on to receive two nominations at the BAFTA Television Awards. Patel resumed his role as Anwar for the second series of Skins, which aired in early 2008.

In August 2007, Patel was cast in the role of Jamal Malik, the central character in Danny Boyle's independent film Slumdog Millionaire. Patel, who appeared on The Today Show with co-star Freida Pintoon 11 November 2008, stated he was chosen for the role of Jamal due to his "average Joe" appearance. He also stated that it was Boyle's daughter, a fan of Skins, who initially recommended Patel for the role. The character Jamal Malik, the protagonist, is an Indian Muslim boy born and raised in the poverty of Mumbai, India. Boyle considered hundreds of young male actors, although he found that Bollywood leads were generally "strong, handsome hero-types", not the personality he was looking for. Boyle's 17-year-old daughter Caitlin pointed him to the British television ensemble drama Skins, of which Patel was a cast member. After five auditions for the role, the actor was eventually cast in August 2007. To prepare for the role, Patel went along with Boyle while scouting for filming locations, where he was able to observe the Dharavi slums for himself. He also reported having had a brief internship at a call centre and working in a hotel, where he spent a day washing dishes and generally observing life in Mumbai.

After the release of Slumdog Millionaire at the end of 2008, Patel went on to receive a number of awards for his performance, including a British Independent Film Award, National Board of Review (NBR) Award, Chicago Film Critics Association Award, and two Black Reel Awards for Best Actor and Best Breakthrough Performance. On 18 December 2008, it was announced that Patel was also nominated for Best Supporting Actor at the 2009 Screen Actors Guild (SAG) Awards. The award eventually went posthumously to the late Heath Ledgerfor his performance in The Dark Knight, though Patel did win the Screen Actors Guild Award for Outstanding Performance by a Cast in a Motion Picture, which he shared with ten other cast members from Slumdog Millionaire. On 8 January 2009, Patel was honoured with the Critics' Choice Award for Best Young Performer. He was also nominated for two London Critics Circle Film Awards, the NAACP Image Award for Outstanding Supporting Actor, and the 2009 BAFTA Award for Best Leading Actor. The film itself won four Golden Globes, including Best Drama Film, and eight Academy Awards.

In 2008, Patel auditioned for a role in M. Night Shyamalan's The Last Airbender, a feature film adaptation of the animated series Avatar: The Last Airbender. In February 2009, it was announced that Patel has replaced Jesse McCartney for the role of Prince Zuko, after McCartney's tour dates conflicted with a boot camp scheduled for the cast to train in martial arts. Shyamalan stated that Patel was "already one of the guys I was interested in. Then I saw Slumdog Millionaire, and the kid just grew in my eyes".