As war begins to spread across Europe, Thomson is energized for  a more direct role.  He resigns from the DA's office and leaves Bob in New York while he sails to Europe to help the relief effort.  En route, the ship is sabotaged and sank, killing everyone aboard except Thomson who survives clinging to the wreckage.  He resolves to face the Axis more directly in a new costumed identity - Mr. America - and returns to the U.S. to track down the saboteur.   Dying his hair black, he decides to allow people to believe Thomson had died so he can operate more covertly and discovers the saboteur is actually a double agent in thw War Department who he promptly brings to justice.  He then lets Bob Daly know that he still lives and Daly resolves to act as his man Friday (Action Comics #33).  Mr. America then begins a war on home front saboteurs and axis sympathizers, often leaving calling card - a red, white and blue feather.  He also faces the return of the Gorrah targeting New York libraries and who is killed when his own bomb drops right in front of him (Action Comics #38). 

In the Summer of 1941, the increasing amount of work demanded a more robust approach.  Using chemical processing of unknown origin, Thomson managed to transform his cape into a mode of flight, essentially a flying carpet that he could use to transport himself to crime scenes quickly,  At the same time, Bob Daly, frustrated by his lack of a role in Mister America's Adventures creates a somewhat ridiculous identity for himself as Fat-Man.  Fat-Man attempt to investigate crimes and stumbles across a new crimelord, surprising a woman with code-name Queen Bee.  The Queen Bee's men capture Daly who is soon rescued by Mr. America but the Queen Bee escapes (Action Comics #42).   The Queen Bee conducts an extensive crime spree over the next several months before it is revealed that she is the victim of a psychological experiment gone wrong, one that inhibits her conscience, conducted by her father.  When he catches up with her, Mister America captures her and they reverse the process with the Queen Bee with no memory of her life of crime (Action Comics #46-49).  Shortly thereafter, with war-fever heating up, Thomson began to use the code-name Americommando (Action Comics #42).  He also was recruited as a member of the All-Star Squadron in the spring of 1942 (All-Star Squadron #31) but his work with them, if extensive, is unrecorded.

In  the winter of 1942, Thomson reads a newspaper article attempting to lure him into a meeting.  Suspecting a trap, Mister America investigates anyway and is quickly attacked.  Fat-Man, ignoring instructions to remain behind, bumbles into the scene and reveals that the assailants are actually American soldiers conducting a deception to take the heroes to a secret meeting.  The meeting turns out to be the President of the United States, who send Fat-Man home and explains to Thomson that America needs an operative deep within Axis territory.  Thomson agrees and undergoes weeks of grueling training to create a new identity - Hauptmann Otto Riker, a Nazi military officer.  Once completed, the newly charged Americommando departs for Berlin (Action Comics #54).  The activities of Bob Daly after this point are unknown.

Upon arriving in Berlin, Thomson sets up contacts with the local resistance and begins a campaign of sabotage, feeding them information he obtains as Hauptmann Riker (Action Comics #55).  He soon becomes aware of a special Axis operatives, a half-German/half-Japanese dwarf named Dr. Ito.  Ito coordinates a wide variety activities from war planning to concentration camps and becomes specifically interested in the activities of Americommando.  Ito ultimately discovers Thomson's ruse as Riker and blows his cover (Action Comics #56-57).  The Americommando then becomes part of the underground taking part in actions in Greece (Action Comics #60), Holland (Action Comics #61) and Russia  (Action Comics #64) among others.  On one case in Romania, he again encounters Dr. Ito but in direct combat flings the diminutive agent from a train, evidently killing him (Action Comics #62).

In the autumn of 1943, the Americommando moves his activities to Asia, operating in Burma (Action Comics #66) and China (Action Comics #69-70) before moving into Japan (Action Comics #71).  He adopts the stolen identity of Captain Brand to infiltrate the German Embassy where he learns of the arrival of the still-quite-alive Dr. Ito (Action Comics #72).  With the Japanese war effort failing, Ito develops a plan to bomb America directly.  He discovers the true identity of "Captain Brand' and takes him into custody, intending to bind him to the bomb as it drops.   Leaving the Americommando bound to the bomb, Ito does not see him escape and begin to wreak destruction on the plan.  Encountering Ito, they two struggle with Ito managed to get off a shot and graze Thomson, who falls unconscious on the bomber controls and open the bay doors, dropping the bombs and Ito into the Pacific.  Hours later, the plane's autopilot takes it over California where Thomson comes to and realizes that the Americommando has finally come home (Action Comics #73-74).

The later activities of Tex Thomson, the Americommando of Earth-Two, are unrevealed.