Justice Brett M. Kavanaugh jumped right into his first day of work Oct. 9, joining his new colleagues in back-to-back oral arguments as they wrestled once again with the meaning and implications of the oft-litigated Armed Career Criminal Act.

In his first high court outing, Kavanaugh showed no signs of shyness—or of the partisanship that marked the hearings into allegations of sexual assault against him. Looking comfortable during the technical but lighthearted two-hour session, he firmly but respectfully asked several questions of lawyers representing both the government and the convicts trying to reduce their stiff sentences.

The 1980s-era...