John's paternal grandparents, Leo Krasinski and Regina L. Wajozynski, were of Polish descent. His maternal grandparents, Edward Joseph Doyle and Eileen Goggin Burke, were of Irish ancestry.
John Krasinski was born on October 20, 1979, in Boston, Massachusetts, United States. He was born to Doctor Ronald Krasinski and Mary Clare. He has two older brothers: Kevin and Paul.