The story goes that it was difficult, rather impossible, for any outsider to extract any information he did not want to reveal.
"Even Mrs Gandhi would say, 'No matter how hard one tries, they can never get a word out of Pranab. All they will see is the smoke coming out of his pipe'," laughed journalist and long-time friend Jayanta Ghoshal, who has known Mukherjee since 1985, as he remembered the immense faith between the prime minister and her finance minister.
As the 13th president of India leaves the Rashtrapati Bhavan on Monday, making way for his successor Ram Nath Kovind, old friends recalled with fondness the many anecdotes that marked his long career in politics.
Mukherjee is said to have had loved his pipes, even after he quit smoking years ago.
"He never smoked cigarettes, only the pipe. After he was asked to quit smoking for health issues, he wouldn't smoke but would keep the pipe in his mouth, without any nicotine, and chew the stem of pipe, just to get the feel of it," Ghoshal told PTI.
A proud owner of over 500 smoking pipes gifted to him by different heads of states and other foreign dignitaries, Mukherjee has donated the collection to the Rashtrapati Bhavan Museum.
His first pipe, Ghoshal said, was given to him by senior Congress leader Debkanta Barua from Assam.
The journalist first met Mukherjee in 1985, at the latter's south Calcutta residence in Southern Avenue, as a junior reporter in the Bengali daily Bartaman.
Young and enthusiastic, Ghoshal would accompany the politician on each of his trips to different districts in West Bengal, gaining political knowledge on the way.
"He was a very scholarly person. Even though I was only a junior reporter, he would explain things (political science) to me and I would listen to him like a student. He would also sometimes share anecdotes about Indira Gandhi. He used to miss her a lot," Ghoshal recalled, remembering the days after Gandhi's assassination when there was a leadership struggle in the party.
Senior Congress leader and former union minister Shivraj Patil describes his long-time colleague as a man who "knew the politics and economics of the country in the best possible manner".
"He was one of the senior most members in parliament and he knew very well in what manner a minister should conduct himself. He knew how the Constitution is to be protected without creating problems for the government."
"He was one of the best presidents of India," Patil added.
For fellow Bengali and Trinamool Congress member Saugato Roy, Mukherjee was a lover of "old political history" and an extremely affectionate and hard working man.
"Although it has been long since I split from the Congress, I continue to enjoy his affection. He loved to talk about old political history. I have known him for 30-35 years and he is an extremely hard working person. When he was the finance minister, I remember how he wouldn't ever leave without meeting the last visitor," Roy said.
Having held several ministerial portfolios in the Indian government, including defence, finance and external affairs, Mukherjee was, as Ghoshal noted, "perpetual Number 2".
"Becoming the President of India gave him the opportunity to become Number 1," he said.