In 1968, he was appointed as a Judge of the Kerala High Court. In 1970 he was appointed as a member of the Law Commission.
Krishna Iyer was the Chairman of the Committee constituted by the Government of India to submit proposals to draw a scheme for extending legal aid to the weaker sections of the society. He was appointed as a judge of the Supreme Court in 1973 and continued as the same till 1980.

Krishna Iyer took initiative to reform the prison laws. Advocating kindness, he supported the rehabilitation of the prisoners and suggested free legal help for the accused in custody.
The verdict of the Ratlam Municipality case highlights the legal responsibility of the local body institutions in health care and sanitation.

According to the service rules of Indian Foreign Service, the women employees had to produce prior approval certificate from the government before marriage. Krishna Iyer made it clear that it is discrimination against women and against the freedom of equality.
He was involved in the verdict of a critical case in the Indian political history. In the Indira Gandhi election case, with certain conditions, he stayed the High Court order, which prevented Indira Gandhi from contesting the elections for six years.