Pratt was born in Brooklyn, New York on November 21, 1871. He was the fourth of six children of the Standard Oil industrialist Charles Pratt, and Mary Helen (née Richardson) Pratt (1840–1907), his father's second wife. His siblings included Frederic B. Pratt, George Dupont Pratt, Helen Pratt, John Teele Pratt (husband of Ruth Baker Pratt, the first woman elected to the U.S. House of Representatives from New York) and Harold I. Pratt. From his father's first marriage to Lydia Richardson (the elder sister of his mother), who died young in 1861, his older half-siblings were Charles Millard Pratt and Lydia Richardson Pratt.