Cindi Leive, the editor in chief of Glamour, says that a writer has about three seconds to grab a reader’s attention, so let’s blurt this out: After 16 years, she says, she’s quitting her job.

This departure, one she announced to her staff this morning, makes her the fourth editor of a major magazine to vacate the role in the space of a week. In order of announcement, they are Graydon Carter of Vanity Fair, Roberta Myers of Elle and Nancy Gibbs of Time.

“As in all things magazine related, damn Graydon got there first,” Ms. Leive said.

She laughed and tucked her bare feet under her on the living room sofa in her Brooklyn townhouse. This was the day before she would inform Glamour’s staff, and she was a little nervous about telling her team the news. “I’m sure I will be, in my grandmother’s words, ‘highly verklempt.’ I’m a bit of a crier anyway,” she said.

She refers to her staff as her “Glam fam”: “I just love love love the people I work with,” she said. Her toenails were painted a bluish gray. They looked perfectly nice but still ready for a fresh pedicure. She will stay on at the magazine until the end of the year, but soon she’ll have the time for primping.

As every magazine editor knows, three is a trend. So what to make of four? Ms. Leive’s departure from Glamour would matter in any circumstance. Coming now, it cements a sudden sense that there is an unprecedented change of the guard. Ms. Leive was among a now fairly thin rank of those who were magazine editors before magazines became brands.

“To me, a brand was Kellogg’s,” she said of her early days. “But I have gotten comfortable with the term.”

Like any media executive worth her six-figure Twitter following, Ms. Leive is proud to share today’s measure of magazine success: 11 million monthly unique visitors to Glamour.com, 15 million followers across social media platforms, a robust “Women of the Year” award ceremony and events business, a video that has garnered 147 million views on Facebook. It’s called “Your Period in 2 Minutes.” “Perhaps you saw it?” she asked.

Ms. Leive’s stomach growled beneath her flouncy Tanya Taylor dress, with its cutout shoulders. She drank water. “I’m leaving the brand in great shape,” she said.

The least vague reason she would offer for her decision to quit now related to her mother, a biochemist who died when Ms. Leive was 19. “Not to get too emo, but my mom died when she was 49 and last year I turned 49,” she said, and here, her voice got wobbly. “I felt like I have been given this gift of so much more life and I wanted to do something with it.”

She wouldn’t comment on what her next gig will be other than to say what it won’t be: “I’m not going to another big media job or to a similar position at another company.” She gave the impression that she has plans. “I adore my kids, but I’m not leaving to spend more time with my kids,” she said.

Ms. Leive and her husband, Howard Bernstein, a film producer, have two children, Lucy, 14, and Ike, 12. They needed some reassurance that their mother leaving her job is a good idea. “When I told this to my son, his main concern was, ‘Oh my God, are you not going to be verified on Instagram anymore?’” she said.

She too was captivated by a certain idea of status when she was younger. While a college student, Ms. Leive was an intern at The Paris Review, working out of its old basement offices. She saw herself studying for a Ph.D. and getting a job that would be, as she put it, “Important with a capital I.”

After graduating from Swarthmore College when she was 21, she allowed herself a one-year frivolity, a job at a fashion magazine.

But then something unexpected happened. She found that she loved being an editorial assistant at Glamour, loved the mix of fashion and politics, civics and silliness. She stayed for 11 years. “The type of things they were writing and editing,” she said of Glamour’s more senior staff, “were the things me and my friends were talking about.” She loved working for Ruth Whitney, Glamour’s editor of 31 years.

She even loved working for Bonnie Fuller, the less refined, more sex- and celebrity-focused replacement to Ms. Whitney. (She says the “three second” attention rule is a Bonnieism.) After working for Ms. Fuller for less than a year, she became the editor of Self, also published by Condé Nast.

When Ms. Fuller was asked to resign in 2001, Ms. Leive returned to Glamour, as its editor in chief.

Even though she had been at Glamour for years before, and also had two years as editor of Self under her belt, there was a steep learning curve. Early in her tenure, she promised a celebrity that she could have approval over her appearance on the cover. “It was someone I greatly admired,” Ms. Leive said.

Ms. Leive selected the photograph that she knew was the right one for the magazine’s demographic and newsstand appeal. The actress hated it. Ms. Leive tried to persuade her that it was a flattering picture, but the actress wouldn’t budge and Ms. Leive worried about making the celebrity unhappy. She didn’t want Glamour to get a bad reputation with Hollywood publicists. But she was positive that she had selected the right photo.

“At a certain point in the interaction, I had to let go of this desire to make this person like me and remind myself that my allegiance was with Glamour and with what I thought was the right cover,” she said. “So I just went for it and said, ‘I’m sorry but this is the cover we’re going to run.’” Ms. Leive said the actress took it in stride and that she never again offered anyone cover approval.

Figuring out how to leverage the internet wasn’t easy, either. “I had what I thought was the most brilliant idea,” she said of a notion that occurred to her around 2010. Glamour could use its famous “Dos and Don’ts” rubric to encourage readers to submit photographs of people wearing the good or the ugly. Visitors to the website would be encouraged to weigh in.

There was a huge influx of traffic, Ms. Leive said, but not just that. “I actually opened Pandora’s box for a troll convention to take place on Glamour.com,” she said. She and her staff decided to hit delete.

She also learned that not every story she thought was splashy would actually make a splash. For the July 2015 issue, Kim Kardashian West appeared on the cover of Glamour, addressing publicly her feelings about the gender transition of her family member Caitlyn Jenner. Ms. Leive and her staff planned a full press rollout.

“Literally an hour before we were putting it out, Vanity Fair’s Caitlyn Jenner cover comes out,” Ms. Leive said. “So my scoop didn’t look that great, and all of a sudden I was getting angry letters from people wanting to know why I was referring to Caitlyn Jenner as Bruce in the magazine.” She laughed, sounding amused and still a little exasperated.

There is much from her long tenure that makes Ms. Leive proud, she said. There is the “Women of the Year” awards, which have honored people including Malala Yousafzai and Gabby Giffords. Ms. Leive oversaw the launch of the Girl Project, which provides resources to support education for girls in more than 100 countries. She published an essay last summer by Barack Obama, called “This Is What a Feminist Looks Like.”

She also made Hillary Clinton the first presidential candidate to be endorsed by Glamour. Ms. Leive says Anna Wintour, Condé Nast’s artistic director and Ms. Leive’s boss, deserves much of the credit for that. When Ms. Leive broached the idea with her, she said that Ms. Wintour said, “If you think this is what is right for your audience and for women, then you should do it.”

All this is why Ms. Leive bristles at the suggestion that her decision to leave magazines, amid something that is approaching a brain drain of longtime magazine honchos, signals that glossies are losing their sheen.

Not that she’s complaining about being in the company of Ms. Myers, Ms. Gibbs and Mr. Carter. “That’s a pretty nice outgoing class to be in, I’ll take it,” she said. “We can all hang out in the corner booth somewhere.”

Only if a former editor in chief can still snag the corner booth, she is reminded.

“What if I can’t get a corner booth anymore, good point!” she said, and cracked up. “We’ll have to meet on my back deck, fending off the dog.”