Armstrong debuted as a pro on July 28, 1931, a fight in which he lost by knockout in the third round against Al Iovino. Like Alexis Argüello, Bernard Hopkins and Wilfredo Vázquez, Armstrong is one of the many world champions who started his career with a loss in his record.
His first victory came a few months later against Sammy Burns, by decision of the judges in the sixth round. Armstrong decides to settle in Los Angeles in 1932, where he began losing two consecutive bouts to four rounds by arbitration, against Eddie Trujillo and Al Greenfield. However, from that moment on he won eleven consecutive victories, a good run until 1933, when he lost again, this time against Baby Manuel. After this new defeat, Armstrong remained unbeaten during 22 battles, achieving, therefore, the record of 17-0-5.
Henry Jr. is part of a select group of boxers who have won championships in three or more different divisions (at a time when there were only 8 world titles universally recognized), in addition to having the distinction of being the only boxer with three championships simultaneous, featherweight, lightweight and welterweight in 1938. 2 Armstrong defended his welterweight title nineteen times.
In 2007, The Ring placed Armstrong as the second best fighter of the last 80 years. Bert Sugar also considered him the second best fighter of all time.