Skilled in Karnatic music, Western and Hindustani classical music and the Qawwali style of Nusrat Fateh Ali Khan, Rahman is noted for film songs amalgamating elements of these and other genres, layering instruments from different musical idioms in an improvisational style. Symphonic orchestral themes accompany his scores, occasionally using a leitmotif. During the 1980s Rahman recorded monaural arrangements in Common with his musical predecessors, K. V. Mahadevan and Vishwanathan–Ramamoorthy. In later years his methodology changed, as he experimented with the fusion of traditional instruments with new electronic sounds and Technology.

Studying in Madras, Rahman graduated with a diploma in Western classical music from the school. Rahman was introduced to Qadiri tariqa when his younger sister was seriously ill in 1984. His mother was a practicing Hindu. At the age of 23, he converted to Islam with other members of his family in 1989, changing his name to Allahrakka Rahman (A. R. Rahman).

Rahman initially composed scores for documentaries and jingles for advertisements and Indian television channels. In 1987 Rahman, then still known as Dileep, composed jingles for a line of watches introduced by Allwyn. He also arranged the jingles for some advertisements that went on to become very popular, including the popular jingle for Titan Watches, in which he used the theme from Mozart's Symphony no.25.

With an in-house studio (Panchathan Record Inn in Chennai), Rahman's film-scoring career began during the early 1990s with the Tamil film Roja. Working in India's film industries, international cinema, and theatre, Rahman is one of the best-selling recording artists, with an estimated 200 million units sold. In a notable two-decade career, he has been acclaimed for redefining contemporary Indian film music and contributing to the success of several films. Rahman has also become a notable humanitarian and philanthropist, donating and raising money for a number of causes and charities. In 2017, Rahman made his debut as a Director and Writer for the film Le Musk.

Rahman's film career began in 1992 when he started Panchathan Record Inn, a recording and mixing studio in his backyard. It would become the most-advanced recording studio in India, and arguably one of Asia's most sophisticated and high-tech studios. Cinematographer Santosh Sivan signed Rahman for his second film Yoddha, a Malayalam film starring Mohanlal and directed by Sivan's brother Sangeeth Sivan that released in September 1992.

Rahman was the 1995 recipient of the Mauritius National Award and the Malaysian Award for his contributions to music, and was nominated for a Laurence Olivier Award for his first West End production. A six-time National Film Award winner and recipient of six Tamil Nadu State Film Awards, he has fifteen Filmfare Awards and sixteen Filmfare Awards South for his music. Rahman has received a Kalaimamani from the Government of Tamil Nadu for excellence in the field of music, musical-achievement awards from the governments of Uttar Pradesh and Madhya Pradesh and a Padma Shri from the Government of India.

Rahman has also been involved in non-film projects. Vande Mataram, an album of original compositions released for India's 50th anniversary of its independence in 1997, is one of India's bestselling non-film albums. He followed it with an album for the Bharat Bala–directed video Jana Gana Mana, a collection of performances by leading exponents and artists of Indian classical music. Rahman has written advertising jingles and orchestrations for athletic events, television and Internet media, documentaries and short films, frequently using the Czech Film Orchestra and the Chennai Strings Orchestra.

In 1999, Rahman partnered with choreographers Shobana and Prabhu Deva and a Tamil film-dancing troupe to perform with Michael Jackson in Munich, Germany at his Michael Jackson and Friends concert. In 2002 he composed the music for his first stage production, Bombay Dreams, which was commissioned by Andrew Lloyd Webber. The Finnish folk-music band Värttinä collaborated with Rahman on the Toronto production of The Lord of the Rings, and in 2004 he composed "Raga's Dance" for Vanessa-Mae's album Choreography (performed by Mae and the Royal Philharmonic Orchestra).

Rahman attracted a Japanese audience with Muthu's success there. His soundtracks are known in the Tamil Nadu film industry and abroad for his versatility in combining Western classical music, Carnatic and Tamil traditional and folk-music traditions, jazz, reggae and rock music. The Soundtrack for Bombay sold 15 million copies worldwide, and "Bombay Theme" would later reappear in his Soundtrack for Deepa Mehta's Fire and a number of compilations and other media. It was featured in the 2002 Palestinian film Divine Intervention and the 2005 Nicolas Cage film, Lord of War. Rangeela, directed by Ram Gopal Varma, was Rahman's Bollywood debut. Successful scores and songs for Dil Se.. and the percussive Taal followed. Sufi mysticism inspired "Chaiyya Chaiyya" from the former film and "Zikr" from his Soundtrack album for Netaji Subhas Chandra Bose: The Forgotten Hero (which featured elaborate orchestral and choral arrangements).

Rahman is involved with a number of charitable causes. In 2004 he was appointed as the global ambassador of the Stop TB Partnership, a WHO project. Rahman has supported Save the Children India and worked with Yusuf Islam on "Indian Ocean", a song featuring a-ha keyboard player Magne Furuholmen and Travis Drummer Neil Primrose. Proceeds from the song went to help orphans in Banda Aceh who were affected by the 2004 Indian Ocean Tsunami. He produced the single "We Can Make It Better" by Don Asian with Mukhtar Sahota. In 2008 Rahman opened the KM Music Conservatory with an audio-media education facility to train aspiring Musicians in vocals, instruments, music Technology and sound design. The conservatory (with prominent Musicians on staff and a symphony orchestra) is located near his studio in Kodambakkam, Chennai and offers courses at several levels. Violinist L. Subramaniam is on its advisory board. Several of Rahman's proteges from the studio have scored feature films. He composed the theme music for a 2006 short film for The Banyan to aid poor women in Chennai.

His first Soundtrack, for Roja, was listed on Time's all-time "10 Best Soundtracks" in 2005. Film critic Richard Corliss said that the composer's "astonishing debut work parades Rahman's gift for alchemizing outside influences until they are totally Tamil, totally Rahman", and his initial global success is attributed to the South Asian diaspora. Music Producer Ron Fair considers Rahman "one of the world's great living composers in any medium".

In 2006, he received an award from Stanford University for his contributions to global music. The following year, Rahman entered the Limca Book of Records as "Indian of the Year for Contribution to Popular Music". He received the 2008 Lifetime Achievement Award from the Rotary Club of Madras. In 2009, for his Slumdog Millionaire score, Rahman won the Broadcast Film Critics Association Award, the Golden Globe Award for Best Original Score, the BAFTA Award for Best Film Music and two Academy Awards (Best Original Score and Best Original Song, the latter shared with Gulzar) at the 81st Academy Awards.

In 2008 Rahman and noted percussionist Sivamani created a song, "Jiya Se Jiya", inspired by the Free Hugs Campaign and promoted it with a video filmed in a number of Indian cities for a cause.

His score for his first Hollywood film, the 2009 comedy Couples Retreat, won the BMI London Award for Best Score. Rahman's music for 2008's Slumdog Millionaire won a Golden Globe and two Academy Awards (a first for an Asian), and the songs "Jai Ho" and "O... Saya" from its Soundtrack were internationally successful. His music on 2008's Bollywood Jaane Tu... Ya Jaane Na was popular with Indian youth; that year, his score and songs for Jodhaa Akbar won critical acclaim, a Best Composer Asian Film Award nomination and IIFA awards for best music direction and score.

He has received honorary doctorates from Middlesex University, Aligarh Muslim University, Anna University in Chennai and Miami University in Ohio. The Composer has won two Grammy Awards: Best Compilation Soundtrack Album and Best Song Written for Visual Media. Rahman received the Padma Bhushan, India's third-highest civilian honour, in 2010.

His work in 127 Hours won him Golden Globe, BAFTA, and two Academy Award nominations (Best Original Music Score and Best Original Song) in 2011. Rahman is an Honorary Fellow of the Trinity College of Music.

In Summer 2012 Rahman composed a Punjabi song for the London Olympics opening ceremony, directed by Danny Boyle, part of a medley showcasing Indian influence in the UK. Indian musician Ilaiyaraja's song from the 1981 Tamil-language film Ram Lakshman was also chosen for the medley.

In December 2012 Rahman and Shekhar Kapoor launched Qyuki, a networking site which is a platform for story Writers to exchange their thoughts. Cisco invested ₹270 million in the startup, giving it a 17-percent share. Qyuki uses Cisco's cloud infrastructure for the site. On 20 December he released the single "Infinite Love" in English and Hindi, commemorating the last day of the Mayan calendar to spread hope, peace and love. Rahman's 2013 tour, Rahmanishq, was announced on 29 July 2013 in Mumbai. Beginning in Sydney on 24 August, the tour moved to a number of cities in India.

On 24 October 2014 Rahman was awarded an honorary doctorate from Berklee College of Music during a concert paying tribute to his music featuring an international cast of students. Upon receiving his award he commented that being honored by Berklee illustrates how his life has come full circle, as at the start of his career, he had planned to study at Berklee before being offered the opportunity to score Roja. During his 7 May 2012 acceptance speech of his honorary doctorate from Miami University in Ohio, Rahman mentioned that he received a Christmas card from the family of the President of the United States and an invitation to dinner at the White House. A street was named in his honour in Markham, Ontario, Canada in November 2013.

On 4 October 2015, the government of Seychelles named A. R. Rahman Cultural Ambassador for Seychelles in appreciation of the "invaluable services contributed to enhance Seychelles' Arts and Culture development."

In January 2016, after a long break A. R. Rahman performed live in Chennai and for the first time in Coimbatore & Madurai, with a complete Tamil playlist. As the name suggests, Nenje Yezhu (which means rise up) began 2016 with a positive note and with music from the heart. The proceeds of this concert will be used for flood relief in Tamil Nadu and also for creating awareness against cancer, supporting VS Medical Trust outside Chennai.

He was interviewed by Arnab Goswami of Republic TV on 9 September 2017 for his outstanding achievements.