Nehru tops the list at 6,130 days, while his daughter Indira Gandhi remains the second longest-serving prime minister with 5,829 days in office.
According the sources, Singh did not celebrate the day and followed his routine schedule.
"The main functions he celebrates related to his office are the anniversaries of the UPA government," said the source.
Manmohan Singh moved to the third slot on Aug 11, replacing his predecessor Atal Bihari Vajpayee, who had been in office for 2,272 days over three terms.
The Manmohan Singh-led UPA-1 (2004-2009), at 1,826 days, holds the second position among the longest-serving central governments.
After Nehru, Manmohan Singh is the only Prime Minister to have been re-elected on completion of a full five-year-term.