Every time Mindy Grossman looks in the mirror, she is reminded that “you are skinny, you are fabulous, you are clever”. It is not some magic talking looking glass, but an engraved compact-sized mirror designed by a friend and stuck to the back of her smartphone. Two of the three mantras are wearing away, leaving only “you are clever”.

“It’s the most important one, anyway,” says the businesswoman tasked with reinventing Weight Watchers for the digital age, where weight loss apps and Insta-fuelled phenomenons like clean eating are in the ascendancy.

“My first Weight Watchers meeting was when I was 14 years old on Long Island, and I went there with my mother,” says Grossman, who has returned 46 years later to run the company. “I’d gained that adolescent weight and wanted to try out for cheerleading … I lost the weight, tried out and made the cheerleading team.”

Grossman’s business credentials (she appeared in the Forbes list of the world’s 100 most powerful women after turning around the Home Shopping Network cable channel) and her immaculate blonde coiffure resulted in the New York Times billing her as the second coming of Jean Nidetch, the Brooklyn-born entrepreneur who founded Weight Watchers in the early 1960s. When we meet, Grossman looks more like a cast member of Sex and the City, with a leopardskin jacket and purple bejewelled Louboutins.

Oprah Winfrey, who owns 10% of Weight Watchers, at the Emmy awards in Los Angeles last month. Photograph: Buckner/Variety/REX/Shutterstock

Grossman arrived at Weight Watchers in July, picking up the baton from Oprah Winfrey, whose purchase of a 10% stake in 2015 triggered a dramatic reappraisal of the brand at a time when it was losing thousands of members. Before Winfrey’s involvement, the writing looked to be on the wall for the 54-year-old brand, with shares changing hands for less than $4 (£3), down from $86 in 2011.

But Winfrey’s patronage is helping the company bounce back, with nearly two years of sales growth now under its belt. In its most recent quarterly sales update, subscriber numbers had surged by 20% to 3.5 million, with the firm growing in both the US and the UK, which account for 70% of its sales.

“She’s human,” says Grossman of Winfrey’s much-publicised battle with her own weight. And she argues that the fortunes of the company are not tied to the billionaire’s waistline. “Oprah’s involvement is not just about losing weight ... she is a visionary businesswoman. She has had an influence globally on how people are thinking about the brand.”

Thanks largely to Winfrey, Weight Watchers shares have regained more than half the value they had lost, making the company worth $2.9bn. But Grossman is tasked with a bigger reinvention as the diet business morphs into the “health and wellness” industry.

“The term ‘dieting’ has some negative associations with consumers, because for some people it reminds them of unsuccessful attempts to lose weight,” says Emma Gubisch, an analyst at Leatherhead Food Research. “It also has the connotation that it is something you do for a short time until you’ve achieved your aim, and then you stop or go back to your old lifestyle.”

Weight Watchers had been losing members for years as people turned to calorie-counting apps and fitness trackers. The brand, whose recent UK faces have included the actor Patsy Kensit and MasterChef’s Gregg Wallace, had begun to feel dated as new stars emerged, such as Joe Wicks, whose Lean in 15 books are now the biggest-selling diet titles of all time.

Grossman says Weight Watchers failed to invest enough in marketing and technology, but insists its holistic package of food plans, advice and community remains powerful in a crowded market where the latest trends include intermittent fasting and the diet drink Skinny Sprinkles.

“[Weight Watchers] have been in people’s lives for a long time, so you are not the new shiny penny out there, but you really are the one that works,” says Grossman of the notoriously faddy diet industry. “When people want to lose weight, they get to a point where they are desperate and looking for the new thing that is going to work miracles. But at the end of the day, there is no such thing.”

With health and wellness now firmly in vogue, Weight Watchers does not want to be seen as a short-term fix – rather, it is positioning itself as a credo to live by, with its app something users check as frequently as Facebook or Twitter. “We’re looking at the future of experiences and how you personalise it,” says Grossman. “We can inspire people to have healthy habits ... that’s not just about losing 10lbs, but creating a structure you can live within.”

The first Weight Watchers cruise, which sailed in May, offers a taste of where the company is headed. The Caribbean trip was a bootcamp designed to shift the pounds, but it took a “wellness” theme, with holidaymakers offered tailored fitness workouts, cooking demonstrations and seminars from experts.

“I’ve been 35lbs heavier than I am now and 25lbs thinner than I am now, and both of those times were probably the most miserable times of my life,” says Grossman. “I’m happier now, because it is about being healthy, not weighing yourself.”

But the big question is: how is Oprah doing? “She looks great,” reports Grossman. “She was at the Emmys wearing a white jumpsuit – and you have to be confident to do that.”