Mahajan started his career as a lawyer in 1913 in Dharamsala, where he spent a year practising. He spent the next four years (1914-1918) as a lawyer in Gurdaspur. He then practiced law in Lahore from 1918 to 1943. During his time there, he served as president of the High Court Bar Association of Lahore (1938 to 1943).
He became a Justice in the pre-independence Punjab High Court. While he was serving there, the Maharaja of Jammu and Kashmir called him to become his Prime Minister for the negotiations regarding merger with India.