Lewis was born in Boston and grew up in Wellesley, Massa­chu­setts.
His parents were physicians – his father a hospital chief of anesthesiology and his mother the head of the Dever State School for intel­lec­tu­ally disabled children.
His father was a World War II veteran and the son of a German Lutheran father and a Russian Jewish mother.
After graduating summa cum laude at the end of the eleventh grade at Boston's Roxbury Latin School he entered Harvard College, where he was for a time a third-string lacrosse goalie.

Lewis has said that he discovered "I wasn't a real math­e­ma­ti­cian [once] I got out of the amateur leagues of high school mathematics", but was "tremendously excited" by the computer-science research oppor­tu­nities at Harvard.

As a senior he lectured a graduate class using a computer-graphics program, SHAPE­SHIFTER, which he had developed for displaying complex-plane trans­for­ma­tions on a cathode ray tube.
SHAPE­SHIFTER automatically recognized formulas and commands hand-entered via a stylus on a RAND tablet, and could be "trained" to recognize the handwriting of individual users.

There being no degree program in computer science per se at Harvard at the time, in 1968 Lewis received his BA (summa, Quincy House) in applied mathematics and was elected to Phi Beta Kappa.

After two years as a mathematician and computer scientist for the National Institutes of Health in Bethesda, Maryland,
he spent a year in Europe as a Frederick Sheldon Traveling Fellow.

He then returned to Harvard, where he earned his M.A. in 1973 and PhD in 1974, after which he was immediately appointed Assistant Professor of Computer Science.
He became an Associate Professor in 1978, and has been Gordon McKay Professor of Computer Science since 1981.

Lewis plans to retire in 2020, at which time a new professorship in Engineering and Applied Sciences, endowed by former student Larry Lebowitz, will be named for Lewis and for his wife Marlyn McGrath, who is Harvard's director of admissions.
Lewis was born in Boston and grew up in Wellesley, Massa­chu­setts. His parents were physicians – his father a hospital chief of anesthesiology and his mother the head of the Dever State School for intel­lec­tu­ally disabled children. His father was a World War II veteran and the son of a German Lutheran father and a Russian Jewish mother. After graduating summa cum laude at the end of the eleventh grade at Boston's Roxbury Latin School he entered Harvard College, where he was for a time a third-string lacrosse goalie.
Lewis has said that he discovered "I wasn't a real math­e­ma­ti­cian [once] I got out of the amateur leagues of high school mathematics", but was "tremendously excited" by the computer-science research oppor­tu­nities at Harvard.
As a senior he lectured a graduate class using a computer-graphics program, SHAPE­SHIFTER, which he had developed for displaying complex-plane trans­for­ma­tions on a cathode ray tube. SHAPE­SHIFTER automatically recognized formulas and commands hand-entered via a stylus on a RAND tablet, and could be "trained" to recognize the handwriting of individual users.
There being no degree program in computer science per se at Harvard at the time, in 1968 Lewis received his BA (summa, Quincy House) in applied mathematics and was elected to Phi Beta Kappa.
After two years as a mathematician and computer scientist for the National Institutes of Health in Bethesda, Maryland,
he spent a year in Europe as a Frederick Sheldon Traveling Fellow.
He then returned to Harvard, where he earned his M.A. in 1973 and PhD in 1974, after which he was immediately appointed Assistant Professor of Computer Science. He became an Associate Professor in 1978, and has been Gordon McKay Professor of Computer Science since 1981.
Lewis plans to retire in 2020, at which time a new professorship in Engineering and Applied Sciences, endowed by former student Larry Lebowitz, will be named for Lewis and for his wife Marlyn McGrath, who is Harvard's director of admissions.
Harry R. Lewis