Mr Singh, who is known for keeping his birthday a low key affair, started his day by attending the 70th foundation day of Council for Scientific and Industrial Research (CSIR) in New Delhi.
Mr Singh, who was born in Gah in Punjab now in Pakistan, says he does not know his actual date of birth but September 26 was given as his birthday by his parents while getting him admitted in a school.
At the CSIR function, he remarked that he shared his birthday with CSIR. "We were both born on 26th September. I can think of no better company than this illustrious gathering of men and women of science, with whom to have my first public engagement on this very special day," he said.
He received greetings from a number of dignitaries and leaders. Among them was Vice President Hamid Ansari who called him up to wish him on his birthday.
This was the first time in several years that Singh was at home on his birthday. On earlier occasions, he would be travelling to or from New York to attend the UN General Assembly.
Last year, he was en route to Frankfurt in Germany on his way back from New York.
On previous such occasions, his birthday was celebrated on board Air India One special flight.
In his first year as the Prime Minister in 2004, he celebrated his birthday mid-air on way to Geneva from New York on Air India's Boeing 747-400.