Mirror, mirror on the wall, who’s the fairest of them all? In the world of 61-year-olds, it’s likely that iconic supermodel and longtime Long Islander Christie Brinkley qualifies. And she’s sharing just how in a new book, “Timeless Beauty” (Grand Central Life & Style; $30).

Brinkley says she decided to write the book after gracing the cover of People magazine in a bathing suit at the age of 60, which, she notes, was not her original plan. “I recall saying at 35, this is my last bathing suit cover,” says Brinkley, who has appeared on more than 500 magazine covers. But the People stylists brought a swimsuit, so Brinkley decided to give it a shot. “I kind of tiptoed out wearing it.”

The cover was a smash, bringing on all kinds of questions: “What do you eat? What kind of exercise do you do? What’s your secret?” Brinkley says she started looking at how she managed “challenges in an industry in which I was told I would be chewed up and spit out by the time I was 30. And I decided to write it down.” Those challenges include making a Broadway stage debut in 2011 as Roxy Hart in “Chicago;” running her own vegan skin care and hair extension lines; not to mention being a devoted mother to her children, Alexa, 29, Jack, 20 and Sailor, 17.

Brinkley, a vegan who eats organic and cheats only rarely (a sliver of mozzarella in Italy), shares tips on everything from fashion and skin care (she admits to using fillers, and a bad bout with Botox) to fitness and even recipes — zucchini noodles and tomato sauce, anyone? She doles out advice without a hint of bossiness, instead motivating almost as if she were a caring BFF. “I would feel horrible if my book made women feel pressured. My message is you don’t have to be perfect to be perfect.”

Yeah, yeah, but Christie, do you ever look bad? “Millions of times,” she says, laughing. In the ’80s, while married to Billy Joel, “I did the Boy George thing, complete with fingerless gloves and multiple braids.” Then there was the champion belt buckle she won in a cutting horse competition the size of a dinner plate. “I worked that belt into every outfit, even evening wear. Ugh.”

But bottom line, to steal a phrase, is we’ll have what’s she’s having. Here are some Brinkley secrets for keeping Father Time at bay.

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Skin: Brinkley is adamant about using sunscreen, and swears by smiling (a natural face-lift, she says) but claims certain items such as walnuts, carrots and oysters (the only raw fish she eats) are super foods for radiant skin.

Makeup: Good brushes are key, Brinkley says, but before you apply, “exfoliate, it makes a huge difference.”

Clothes: “I don’t believe in ‘age-appropriate’ dressing. I call it ‘body-appropriate.’ There’s no rule book; bodies come in all sizes and shapes whether you’re 20 or 65. I believe in accentuating the positive.”

Diet: No more “deny-iting” is her mantra. Instead of obsessing over what you can’t have, think about “gifting” yourself with what you can have by making informed choices. “You will be rewarded with more energy, a clear, glowing complexion, shinier hair,” and, she writes, “a trimmer figure.”

And speaking of slim figures, Brinkley’s not keen on weighing herself, but instead has a pair of “honest” jeans. “They’re the pair that fits me perfectly when I’m at my ideal weight. So if I’m ever worried that I may have put on a few pounds, I slip into those. . . . They never lie to me.”