Stewart M. Brandborg, a conservation activist and Wilderness Society leader who helped draft and advocate for passage of the landmark Wilderness Act of 1964 that set aside millions of acres of land for protection from human development, died April 14 at his home in Hamilton, Mont. He was 93.

The cause was congestive heart failure and lung disease, said a daughter, Betsy Brandborg.

Mr. Brandborg, known as “Brandy,” grew up in Montana and Idaho national forests where his father served as supervisor. As a child, he hunted, fished and hiked in the woods, streams and mountains around his home. He came to Washington in the 1950s to work for the National Wildlife Federation and was soon recruited to the Wilderness Society as assistant executive director to Howard Zahniser, who spent nearly eight years fighting for the Wilderness Act.

Mr. Brandborg, at 6-foot-2 and 200 pounds, cut an imposing figure in the halls of Congress, as he helped Zahniser lobby for passage of the bill against powerful timber, mining and grazing interests. He was described in historian James Morton Turner’s 2012 book “The Promise of Wilderness” as “a bear of a man . . . deep voiced, and devilishly charismatic. He could give a busy taxi cabdriver reason to care about wilderness and he could hold the attention of a senator on a street corner.”

He assisted in dozens of drafts and revisions as it wound its way through the legislative process, working out compromises that set the path for a House victory with only one dissenting vote. Zahniser died just months before President Lyndon B. Johnson signed into law the Wilderness Act, which created what Smithsonian magazine called the first time “land was set-aside for the specific purpose of protecting it from the reach of mankind.”

Mr. Brandborg had succeeded Zahniser and remained at the helm of the society for the next 12 years, seeking to bolster the act by obtaining wilderness recommendations from federal agencies.

“These coming years,” he wrote in 1966, “will test our power to the limit: our ability to communicate the need for preserving wilderness; our depth of conviction and willingness to follow through on our commitments as citizens; and above all our basic faith in the American people, who are moving so fast and crowding so closely, and needing wildness so much more today than ever before.”

During his tenure, more than 70 wilderness areas in 31 states were brought under the Wilderness Act’s protection. Since 1964, the National Wilderness Preservation System had expanded almost every year. It now includes 765 wilderness areas covering nearly 110 million acres in 44 states and Puerto Rico.

Stewart Monroe Brandborg was born Feb. 2, 1925, in Grange­ville, Idaho, where his father was supervisor of Nez Perce National Forest. He was 10 when his father was transferred to Bitterroot National Forest in Montana, and they moved to ­Hamilton.

Mr. Brandborg graduated from the University of Montana in 1947. He did research on mountain goats for the state of Montana and later on elk and other big-game species for the state of Idaho. In 1951 he received a master’s degree in wildlife biology from the University of Idaho.

His wife of 64 years, the former Anna Vee Mather, died in 2013. Survivors include five children, Becky Brandborg and Betsy Brandborg, both of Helena, Mont., Dan Brandborg and Fern Schreckendgust, both of Hamilton, and Lisa Orshoski of Elkwood, Va.; seven grandchildren; and two great-grandchildren.

After being fired from the Wilderness Society in a dispute over money-raising techniques, Mr. Brandborg spent four years at the National Park Service as a special assistant to the director. Later he ran regional conferences and developed training materials for leaders in environmental movements. In 1986 he moved to Montana, where he grew up, to continue his work as an environmental proselytizer and defended the Wilderness Act against attempts to permit development in protected areas.

Author and conservationist Terry Tempest Williams once told the Associated Press that she saw poetry in the law’s wording that defines wilderness as an “area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain.”

“Those who wrote this legislation into being understood the crucial and subtle relationship between language and landscape,” she said. “How we speak about wild, open country is closely aligned with how we treat it. Open lands open minds.”