As Col. Prescott was a modest man of fairly humble financial means, there are no formal portraits, busts, etc. [Though, note, there is this.] A wonderful oral description, however, does survive… It is found in Beside Old Hearthstones, available online here.
Some interesting facts are here added in regard to the personality of Colonel William Prescott, given by his grandniece, Mrs. Sarah (Chaplin) Rockwood, to Dr. Samuel A. Green. Her father was Rev. Daniel Chaplin, D.D., of Groton; and her mother was Susanna, eldest daughter of Judge James Prescott, brother of the colonel. She was ten years of age when the hero of Bunker Hill died.
She describes him as a tall, well-proportioned man, with blue eyes and a large head. He usually wore a skull-cap; and he parted his hair in the middle, wearing it long behind, braided loosely, and tied in a club with a black ribbon, as was common in those days. He had a pleasant countenance, and was remarkably social and full of fun and anecdotes. He was dignified in his manner, and had the bearing of a soldier.
Authorities agree on the value of early impressions; and we can but credit this description of the personal appearance of Colonel Prescott, for it was indelibly stamped upon the youthful Sarah Chaplin when sitting upon the knee of the old soldier.
…She attained the remarkable age of one hundred and four years.