Dr. Harold Bornstein, who has been Trump's personal physician for 35 years, told the network "[Trump's] health is excellent, especially his mental health."
Bornstein said he had rushed to finish the letter, which was released in December, while a limo waited outside his Manhattan office.
The letter boasted that the 70-year-old real-estate mogul was in "astonishingly excellent" health.
Bornstein's sit-down with NBC News comes after weeks of speculation from the Trump campaign over Hillary Clinton's health.
Trump surrogates have floated conspiracy theories surrounding the mental health and physical stamina of his Democratic rival - garnering scoffs from Clinton herself this week, who called the speculation a "paranoid fever dream."
In the letter confirming Trump's health, Bornstein wrote, "If elected, Mr. Trump, I can state unequivocally, will be the healthiest individual ever elected to the presidency."
Those statements drew fresh scrutiny this week for their hyperbolic phrasing, to which Bornstein told NBC News, "I like that sentence to be quite honest with you and all the rest of them are either sick or dead," referring to past presidents that he claimed may have sicker than they let on.
"I think I picked up [Trump's] kind of language and then just interpreted it to my own," Bornstein added.
As a fan of his patient, Bornstein said, "I like Donald Trump because I think he likes me."