Terry Sloane was born in the 1910s, a child prodigy from early youth. As a young boy he demonstrated superb skills in athletics, martial arts and engineering skills. He entered college at the age of 12 and graduated in less than a year. Having obtained the maximum formal education available to him, he devoted himself to athletics, again showing superb performance. He later turned his acumen to business, where he rapidly became successful and wealthy. These accomplishments, while considerable, were empty to Sloane, and left him feeling unchallenged and depressed. He resolved to end his life, rather than continue the "perfect" existence. As he was driving home from work, he noticed a young woman standing on the edge of a bridge. Before his eyes, she jumped into the river. Acting quickly, Sloane saved the woman from drowning and learned her tale. She identified herself as Wanda Wilson, a local girl whose younger brother had fallen in with a gang led by a petty criminal called the Big Shot. Sloane adopted the identity of Mister Terrific and found renewed purpose in defeating the petty thug. Sloane then took upon himself to set up an alternative to juvenile delinquency in the "Fair Play Club," an institution that provided urban kids a place to exercise and find shelter from detrimental influences present in Depression-era America (Sensation Comics vol. 1 #1).

As Terry Sloane, Mister Terrific hired Wanda Wilson as his personal assistant; he later became romantically involved with her. Sloane remained active as Mister Terrific throughout the war years, focusing mainly on minor criminal elements in his native city. An extraordinary case included being pulled into the year 7532 by a futuristic council who idolized his exploits and convinced him to thwart the dictator Black Barax (Sensation Comics #28).  In a case as yet unrevealed, Mister Terrific achieved national prominence in when he became reserve member of the JSA in the early to mid 1940's. In 1945 when he joined the Justice Society of America in two cases, one that illustrated the importance of Germany's defeat to a World War II-era pacifist (All-Star Comics vol. 1 #24). The JSA recognized Sloane's talents (1,000 of them, no less) and he was later awarded full membership at some point.

As the war would down, Mr. Terrific began to encounter the novel "super-villain" type criminals that were becoming more common.  In April of 1945, he was touring one his chemical plans in the town of Mudville when a meek pharmacist terrorized by criminals became the monstrous Gorilla Ganty (Sensation Comics #40).  In October of that year, he had a run-in with a failed actor with nonetheless prodigious talents of disguise and distraction who had opted the criminal nom du guerre of Mr. Futile (Sensation Comics #46).  At some point, he encountered the criminal known as Roger Romaine, The Spirit King, though the origins of their first  dust-up on Earth-Two remains unclear. It is believed that Mister Terrific retired from active duty as a costumed adventurer sometime prior to the disbanding of the JSA in 1951.  His activities during the following decade, particularly the fate of girlfriend Wanda Wilson, are unknown.