Devi travelled the world demonstrating her arithmetic talents, including a tour of Europe in 1950 and a performance in New York City in 1976. In 1988, she travelled to the US to have her abilities studied by Arthur Jensen, a professor of psychology at the University of California, Berkeley. Jensen tested her performance of several tasks, including the calculation of large numbers. Examples of the problems presented to Devi included calculating the cube root of 61,629,875 and the seventh root of 170,859,375. Jensen reported that Devi provided the solution to the above mentioned problems (395 and 15, respectively) before Jensen could copy them down in his notebook. Jensen published his findings in the academic journal Intelligence in 1990.

She returned to India in the mid-1960s and married Paritosh Banerji, an officer of the Indian Administrative Service from Kolkata. They were divorced in 1979. In 1980, she contested in the Lok Sabha elections as an independent, from Bombay South and from Medak in part of Andhra Pradesh (now Telangana). In Medak she stood against Indira Gandhi, saying she wanted to "defend the people of Medak from being fooled by Mrs. Gandhi"; she stood ninth, with 6514 votes (1.47% of the votes). Devi returned to Bengaluru in the early 1980s.

In 1977, she wrote The World of Homosexuals, the first study of homosexuality in India. In the documentary For Straights Only, she said that her interest in the topic came out of her marriage to a homosexual man and her Desire to look at homosexuality more closely to understand it.

On 18 June 1980, she demonstrated the multiplication of two 13-digit numbers—7,686,369,774,870 × 2,465,099,745,779—picked at random by the Computer Department of Imperial College London. She correctly answered 18,947,668,177,995,426,462,773,730 in 28 seconds. This event was recorded in the 1982 Guinness Book of Records. Writer Steven Smith said, "the result is so far superior to anything previously reported that it can only be described as unbelievable".

On 4 November 2013, Devi was honoured with a Google Doodle for what would have been her 84th birthday (https://www.google.com/doodles/shakuntala-devis-84th-birthday).

The book, considered "pioneering", features interviews with two young Indian homosexual men, a male couple in Canada seeking legal marriage, a temple priest who explains his views on homosexuality, and a review of the existing literature on homosexuality. It ends with a call for decriminalization of homosexuality, and "full and complete acceptance—not tolerance and sympathy". The book, however, went mostly unnoticed at that time.