The writer and actor, whose next production is a mini-series based on the Titanic disaster, was painted by Daphne Todd.
Ms Todd, who became the first female president of the Royal Society of Portrait Painters in 1994, won last year’s BP Portrait Award with a painting of her 100-year-old mother, depicted shortly after her death.
Lord Fellowes, who was also behind Oscar-winning film Gosford Park, said: “What most impressed me about Daphne’s work method was her complete lack of interest in any photographic record.
“She paints entirely from life. What she has not seen for herself, she will not use, and that somehow gives the finished picture an authenticity that is vivid and even shocking.
“As it happens we do know each other well, but that is not the reason she got such a good likeness.
“It was because of the process in which she carefully and meticulously dissected my face and form with her eyes, and then reconstructed it on the canvas with her extraordinary talent.”
Ms Todd, who painted Lord Fellowes at his home, said she found him “difficult to pin down” but added that he “exuded a massive presence”.
She said: “One might expect an actor to be particularly mercurial. I was therefore surprised to find that Julian Fellowes in default mode was almost disconcertingly still.
“With his smooth baby skin, faint eyebrows and decidedly uncraggy contours it was as difficult to pin down the evidence of his inner character as it is with a young girl. I had to look harder.”
Lord Fellowes will open the private view of the Royal Society of Portrait Painters annual exhibition at The Mall Galleries in Westminster, London, on May 4. The exhibition starts the next day and runs until May 20.
ITV says viewers watching the Titanic mini-series will be “taken on a heart-wrenching journey through Titanic’s last hours, as the drama reveals which of the characters they have come to know so well will survive…and who does not”.
The series will be shown next year to mark the centenary of the liner’s demise in 1912.