He became a vice-president of the bond department of the
Guaranty Trust Company of New York in 1916, eventually spinning the division off into a separate and subsidiary securities company, the Guaranty Company, where he worked in cooperation with
J. P. Morgan, Jr. In 1927, Morgan invited Stanley to become a partner in his firm, replacing
Dwight Morrow, who became the
United States ambassador to Mexico. 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. In 1935, along with
Henry Sturgis Morgan,
J. P. Morgan's grandson and J. P. Morgan Jr.'s son, he helped found Morgan Stanley to take up the securities business that had to be given up by J. P. Morgan, and became the firm's
senior partner when it was reorganized from a corporation to a partnership in 1941. Stanley was influential in his testimony in the 1940s successfully defending the industry against government charges that it was anti-competitive. ==Personal life==