He'd tell lawyers gently, "Let me ask a stupid question," then subject them to an intellectual grilling en route to decisions touching many aspects of American life. The justice prodded the government to take global warming more seriously, and he stood for campaign-finance controls in an imperfect world in which he acknowledged, "Money, like water, will always find an outlet."
Stevens said Friday he's retiring in the summer. That sets up a struggle over his replacement that is likely to be far more contentious than the process 35 years ago that placed him on the high court.
The man who became the court's leading liberal was nominated by a Republican president, Gerald Ford, and confirmed 98-0 by the Senate, a consensus almost unimaginable in this partisan, ideologically driven era.
Stevens' influence waned and waxed over the decades, reaching its height after other liberals retired in the early 1990s. For a dozen years after, the justice in the bow tie proved adept at drawing votes from Republican appointees Sandra Day O'Connor and Anthony Kennedy to frustrate Chief Justice William Rehnquist's conservative agenda of promoting states' rights and the death penalty.
He's had considerably less success swaying the balance under Chief Justice John Roberts, and with the more conservative Justice Samuel Alito in O'Connor's seat. Even so, Stevens notably influenced a 2008 decision allowing terrorism suspects, held without charge for years at the military-run prison at Guantanamo Bay, Cuba, to plead for their freedom in U.S. federal courts.
Stevens had a reputation as a bright and independent federal appeals court judge when Ford nominated him to the Supreme Court.
Early on, he proved idiosyncratic, appearing not to care if other justices went a different way.
Stevens was "the most independently minded and intellectually creative member of the high court, a man of great integrity," said James Simon, former dean of the New York Law School.
"The benefit is that he always offered a fresh perspective. But a downside, perhaps, is that he didn't have much of a following."
You could sometimes tell when Stevens was headed toward a maverick opinion. He would preface it with: "As I understand this case."
Stevens' power grew in the late 1980s and 1990s after moderate conservatives O'Connor and Kennedy joined the court. He found them open to his views on issues such as abortion rights, bringing them aboard by crafting narrow rulings or by using his status as the senior liberal to assign certain opinions to them.
In a major abortion case in 2000, he cobbled together a five-vote majority that upheld Roe v. Wade, the landmark abortion-rights law, and struck down Nebraska's ban on the procedure that critics call partial-birth abortion.
Rather than write the opinion himself, Stevens assigned it to Justice Stephen G. Breyer, then wrote a concurring opinion asserting that abortion opponents were spreading misleading rhetoric.
"The rhetoric is almost, but not quite, loud enough to obscure the quiet fact that during the past 27 years, the central holding of Roe v. Wade has been endorsed by all but four of the 17 Justices who have addressed the issue," Stevens added. Yet in 2007, Roberts and Alito helped form a majority to uphold a federal ban on the same abortion procedure.