By the end of 1925, Ainsworth's growing stature in the field of ordnance won him the position of gunnery officer on the staff of the Commander, Destroyer Squadrons, Asiatic Fleet.

In July 1927, he took command of the destroyer Paul Jones, but left the ship late in the summer of 1928 to begin three years at the Naval Academy as an instructor in the Department of Navigation.

In June 1936, Ainsworth became the executive officer of the battleship Mississippi and, two years later, he became Professor of Naval Science and Tactics at Tulane University in New Orleans, Louisiana.

Ainsworth promptly took that veteran battleship to the Pacific to strengthen the Navy's surface force in that ocean which had been seriously weakened by the Japanese surprise attack on Pearl Harbor.

Its bombardment of the new Japanese air base at Munda on the island of New Georgia would be, in the words of naval historian, Samuel Eliot Morison, "... long regarded as a model...." Transferred to command of TF 18 and Cruiser Division 9 (CruDiv 9), Ainsworth continued his success during a prolonged series of runs up the long, narrow body of water between the central Solomon Islands which American bluejackets had nicknamed "the Slot".

At the end of June, the tempo of American fighting in the southwestern Pacific picked up since the Navy had finally managed to assemble enough amphibious shipping in that theatre to resume the offensive.

About a year later, Ainsworth won the Legion of Merit Medal by his "... exceptionally meritorious conduct ..." while commanding the fire support group during operations which recaptured Guam.