Radio journalist Joshua Johnson will succeed Diane Rehm as the host of the daily public-affairs discussion program that Rehm turned into one of NPR’s most durable franchises over its 37-year run.

Rehm introduced Johnson during her broadcast Wednesday morning from WAMU-FM, the Washington public station that has produced her program since 1979.

Johnson will host a new program, called “1A,” that will have roughly the same format as “The Diane Rehm Show” — interviews, panel discussions and listener phone calls about politics, government, science, the arts and other topics spread over a two-hour block. Johnson said he will tweak the long-running format somewhat, sometimes exploring multiple topics within an hour and occasionally delving into pop culture.

The appointment, by WAMU’s managers and with Rehm’s support, instantly makes Johnson, 36, one of the most prominent figures in public radio. Rehm’s program, which is distributed by NPR, has a passionate and loyal following, with a weekly audience of about 2.6 million. It is carried on some 198 stations nationwide.

“I’m humbled, grateful and excited, but mostly humbled,” Johnson said in an interview this week. He called his appointment “a gift, a treasure, and I’ll treat it as such.”

Johnson, an anchor and host at public radio and TV station KQED in San Francisco, was something of a surprise choice. He was among the lesser-known of the 30 or so candidates whom WAMU interviewed after Rehm said last December that she intended to retire at the end of this year. (Her last live broadcast will be Dec. 23.)

WAMU General Manager J.J. Yore said Johnson wasn’t on the station’s initial list of candidates. But his stock rose after his on-air tryout over two days in late September. “He brought an openness to his sound, a kind of curiosity . . . He was at ease and put his guests at ease,” Yore said.

Johnson was the morning newscaster at KQED for five years, and he stepped down in February to produce and host a series about race called “Truth Be Told.” He was also a substitute host on “Forum,” the station’s nationally syndicated news-discussion program.

A Florida native, Johnson graduated from the University of Miami and worked as an anchor and reporter for TV and radio stations in the state. He joined KQED in 2010.

As a younger African American man, Johnson may help NPR expand its appeal among the younger, male and racially diverse listeners that public radio has struggled to attract throughout its history. Public radio and TV audiences are largely white and aging, a development that has concerned industry leaders for some time.

Yore acknowledged that “diversity was an important factor in creating this new show. It’s not lost on any of us that having a young, African American male leading the conversation on public radio on a national stage is very important.”

Rehm, 80, said her decision to leave her program was “1,000 percent the right decision for me and the station at this time.” But, she added, “I’m feeling somewhat guilty about leaving in this moment” following Donald Trump’s surprising presidential victory. “We need more discussion about what has happened and why this has happened.”

She elaborated on this in a speech she gave last week at a gala in Washington honoring her show. “We’re all going to have to learn to listen in new and different ways, better and harder to those with whom we may not agree, those who are different from us, who may not even be aware of public radio, those who feel they have not been heard,” she said. “We who’ve been in public radio for all these years thought we were being broad and bold. But now we know we’ve been speaking with only a small portion of the electorate and the population at large. We have to do better.”

Rehm said in an interview she wasn’t speaking about Trump supporters specifically, but about people who are “comfortable in their own factual universe” and not open to different points of view. “They want to construct a universe that is within the confines of the way they’re living. Public radio hasn’t reached them.”

Johnson declined to discuss his political views, but he said he was as committed as Rehm to presenting a balanced show. “I see a lot of similarities in how I like to represent myself on the air in the way she presents herself on the air — the same style and ethic of conversation.”

At the same time, he added, “I won’t be the next Diane Rehm. I’m the first Joshua Johnson.”