Adams was born in [[Brooklyn, New York]], to a wealthy family, the son of Elizabeth Harper (née Truslow) and [[William Newton Adams]] Jr.<ref name="auto">{{cite book|title=Who Was Who in American History – the Military|date=1975|publisher=Marquis Who's Who|location=Chicago|isbn=0837932017|page=2}}</ref>
His father had been born in [[Caracas]], [[Venezuela]]. His paternal grandfather William Newton Adams Sr. was American of [[English Americans|English]] descent with roots in [[Virginia]] and his paternal grandmother Carmen Michelena de Salias was a Venezuelan of [[Spanish immigration to Venezuela|VenezuelanSpanish of Spanishdescent]] descent back to [[Gipuzkoa]] and Seville in the eighteenth century and a family from [[Seville]].<ref>{{cite book|url=https://books.google.com/books?id=kzgrDwAAQBAJ&pg=PA4&lpg=PA4&dq=james+truslow+adams+ancestry+spain#v=onepage |title=James Truslow Adams: Select Correspondence|isbn=9781351511186|accessdate=JanuarySeptember 822, 2020|last1=Nevins|first1=Allan|date=5 July 2017}}</ref> The earliest paternal ancestor was Francis Adams from England, an indentured servant who settled in the [[Province of Maryland]] in 1638.<ref>{{cite book|url=https://books.google.com/books?id=kzgrDwAAQBAJ&pg=PA4&lpg=PA4&dq=james+truslow+adams+ancestry+spain#v=onepage |title=James Truslow Adams: Select Correspondence|isbn=9781351511186|accessdate=January 8, 2020|last1=Nevins|first1=Allan|date=5 July 2017}}</ref><ref>{{cite web|url=https://books.google.com/books?id=eEgBAAAAMAAJ&q=James+Truslow+Adams+venezuelan+grandmother&dq=James+Truslow+Adams+venezuelan+grandmother |title=James Truslow Adams: historian of the American dream – Allan Nevins – Google Books |work=Books.google.ca |accessdate=January 8, 2020|last1=Nevins|first1=Allan |year=1968}}</ref><ref>{{cite web|url=https://books.google.com/books?id=ZRAMAQAAIAAJ&q=James+Truslow+Adams+venezuelan+grandmother&dq=James+Truslow+Adams+venezuelan+grandmother |title=Opinions and attitudes in the twentieth century – Google Books |work=Books.google.ca |accessdate=2012-11-26|last1=Morgan |first1=Stewart S. |last2=Thomas |first2=William Henry |year=1938}}</ref><ref>{{cite web|url=https://books.google.com/books?id=9YseAQAAMAAJ&q=Elizabeth+Harper+(Truslow)+and+William+Newton+Adams,+Jr.&dq=Elizabeth+Harper+(Truslow)+and+William+Newton+Adams,+Jr. |title=The New England Historical and Genealogical Register – Google Books |work=Books.google.ca |accessdate=2012-11-26|year=1949 }}</ref>
Adams took his bachelor's degree from the [[New York University Tandon School of Engineering]] (then [[Polytechnic University of New York|Polytechnic Institute of Brooklyn]]) in 1898, and a MA degree from [[Yale University]] in 1900. He entered investment banking, rising to partner in a [[New York Stock Exchange]] member firm.<ref name="auto"/> In 1912, he considered his savings ample enough to switch to a career as a writer.
