William was an inventor with General Electric for whom the Stanley Works building was named in Pittsfield, Massachusetts.

In 1927, Morgan invited Stanley to become a partner in his firm, replacing Dwight Morrow, who became the United States Ambassador to Mexico.

[1] He made his name as a leader of the investment banking industry at J.P. Morgan, making the firm a strong player in securities offerings and especially the bond market, particularly after the Glass–Steagall Act separated commercial and investment banking.

Stanley was influential in his testimony in the 1940s successfully defending the industry against government charges that it was anti-competitive.

In 1940, he led the New York campaign to raise $1.5 million (equivalent to $32,622,000 in 2023) for the United States Commission for the Care of European Children, a private organization providing relief to young war refugees.