[1] He scored music for more than three hundred Malayalam films, many dramas, and twenty Tamil and four Kannada movies.

He performed a number of classical concerts from 1945 to 1962 with multiple accompanists, his usual ones being Chalakudy Narayanaswamy and Mavelikara Krishnankutty Nair.

At the end of his classical concerts, he used to set tunes to the poems of Ulloor Parameswaran Iyer, Kumaranasan, Changampuzha, G. Kumarapilla, O. N. V. Kurup, P. Bhaskaran, amongst many others.

The work that brought him to the limelight was the drama song titled "Ponnarivaal ambiliyil kanneriyunnoole", written by his friend O. N. V. Kurup and composed and sung by himself.

Through his compositions, Devarajan would cast an indelible imprint in the Malayali theatre arena, especially after the famous KPAC drama Ningalenne Communistaakki, written by Thoppil Bhasi in 1952.

His music embraced different styles with the Carnatic and Hindustani melody lines meeting folk idioms and Western harmony.

Despite being a strong atheist, he composed devotional songs like "Harivarasanam", "Guruvayoor Ambalanadayil", "Chethi Mandaram Thulasi", and "Nithyavishudhayam Kanyamariyame", which are considered classics in that genre.

[citation needed] Apart from Vayalar, he has also given tunes to lyrics by other poets and songwriters like O. N. V. Kurup, P. Bhaskaran, Sreekumaran Thampi[12] and Bichu Thirumala.

M. K. Arjunan, R. K. Shekhar, Johnson, Vidyasagar, Ouseppachan, M. Jayachandran, Ilayaraja, A. R. Rahman and many others who later became famous as music directors worked as his assistants, conductors, and instrumentalists.