He predominantly worked in Bollywood and regional film songs since the 1970s, including 150 Hindi and Bengali compositions by R. D. Burman.

After Burman's death in 1994, citing a lack of quality music composition, Kumar withdrew from playback singing and concentrated on live orchestra shows.

At one such function arranged by Bengali actor "Mahanayak" Uttam Kumar, the audience continued requesting encores.

The song was "Hosh Mein Hum Kahan", composed by Sapan Jagmohan for the film Darwaza, which was released in 1978.

He sang in various films, including Aandhi, Aap Ke Deewane, Khatta Meetha, Gol Maal, Des Pardes, Ganga Ki Saugandh, Deewanagi (1976), Duniya Meri Jeb Mein, Parvarish, Humare Tumhare (1979) and Baton Baton Mein.

Then Amit sang all the songs for the 1981 film Love Story and won a Filmfare Award for the duet "Yaad Aa Rahi Hai" with Mangeshkar.

Kumar's singing in "Aao Naye Sapne Bune", composed by Basudev for Main Qatil Hun (1984).

[7] He had several "chart-busters" in 1982 and 1983,[citation needed] and "hits" with the duets "Tu Rootha Toh Main Ro Doongi Sanam", "Gali Gali Dhoonda Tujhe" and solo songs "Halla Gulla Maza Hai" and "Mana Abhi Tu Kamsin" from the film Jawani (1984).

[8] He had solo and duet songs in hit films such as Hum Paanch, Bulundi, Itni Si Baat, Hamari Bahu Alka, Anokha Bandhan, Ustaadi Ustad Se, Bheegi Palkein, Khatron Ke Khiladi, Maalamaal, Ilzaam, Anokha Rishta, Rama O Rama, Jaaydaad, Tridev, ChaalBaaz, Aag Se Khelenge, and Dost.

He became the voice of Kumar Gaurav in the 1980s and their combination gave many hit songs from films such as Romance (1983), Teri Kasam, Lovers (1983), All-rounder and Telefilm-Janam.

In the late 1980s, Kumar sang songs for Anil Kapoor in films including Tezaab, Yudh and Aag Se Khelenge and continued to sing for him in the 1990s.

The Anuradha Pudwal-Amit Kumar duet "Kehdo Ke Tum Meri" and solo song "Ek Do Teen" from Tezaab were part of Binaca Geet Mala's annual list in 1988.

His duet "Yeh Kismat Hai Kya Kisne Dekha Yahan" with Alka Yagnik from Ghar Ka Chirag became popular in 1989.

In this decade, Kumar had notable songs in films such as Sailaab, Police Public, Aaj Ka Arjun, 100 Days, Awwal Number, ChaalBaaz, Khel, Vishwatma, Honeymoon, Aaj Ka Goonda Raaj, Gurudev, Bade Miyan Chote Miyan and Judaai and also had hit songs in the films which flopped at the box office, such as Jawani Zindabad, Afsana Pyar Ka, Indrajeet, Sangdil Sanam, Jaagruti, Deewana Mujhsa Nahin and Suryavanshi.

Their collaborations include Baaghi: A Rebel for Love, Mera Pati Sirf Mera Hain, Maha-Sangram, Swarg, Anjaane Rishtey, Talaashi, Sangdil Sanam, Yaad Rakhegi Duniya, Woh Phir Aayegi, Aaj Ka Goonda Raaj, Sanam, Adharm, and Rakhwala.

Kumar gained the reputation of being a singer who belted out hit songs irrespective of the film being a commercial success or critically panned.

He even worked with Ilaiyaraaja for the song "Yeh Gulabi Shaam Ka Nasha", a duet with Asha from the film Mera Farz (1988).

He also did songs for the television film Janam, and sang the title tracks for the Hindi-dubbed version of Disney's animated series Duck Tales and Bob the Builder.