Twenty-three years ago, a child ran into my life. Her name was Sabrina, and I met her in Paris, when she was five and I was 23. I had been in Paris for two years. As the Paris editor of the Spanish Vogue group of magazines, I had a wonderful career, a cozy domesticity with my sloe-eyed actor boyfriend, Eric Adjani, and a killer wardrobe. It was a charmed time in my life, full of love and glamour. Flowers sent by Karl Lagerfeld, front-row seats at fashion shows, highbrow conversation about the newest trends – I loved it all. Becoming the guardian of a little girl wasn’t part of the plan.

I first noticed Sabrina one chilly March morning while waiting for a taxi to sweep me off to the Vogue office on the sumptuous rue Saint-Dominique. The chic little three-storey house that we had just renovated took up most of the courtyard of my building, but there was some social housing mixed into the block in the way of well-worn French cities, where the luxurious and the miserable knock elbows. Sabrina was playing outside my door, and I was struck by her luminous smile. She had skinny legs, a dirty face and tangled hair. She was in a lightweight dress, not at all suitable for the cold weather.

I looked up and down the street for the adult who should have been supervising her.

“Hello,” I said tentatively. “Where’s your mummy?”

She smiled shyly and looked away.

“Why aren’t you in school?”

She stared down at her shoes, twisting the hem of her dress in her hands.

Something was clearly amiss. I crouched down beside her so we were at eye level.

“Are you hungry?” I asked.

She nodded.

By the time the taxi arrived, Sabrina and I were sitting together companionably on the step, sharing a bowl of early strawberries, with my dog Brioche, a spaniel-setter cross, snuggled between us. I waved the driver off.

‘Brioche was immediately annexed and became her exclusive property’: Lisa and Sabrina with Brioche the dog. Photograph: Lisa Lovatt-Smith

By that summer we were spending every afternoon together. In those early days of getting to know each other, Brioche provided a lot of the glue. Sometimes we’d just sit quietly, me working in the courtyard while Sabrina stroked Brioche’s soft ears. One June day as I was walking past her flat, a small TV soared out of the window and shattered on the cobblestones in front of me. A woman screamed. At that moment Sabrina ran out the front door, practically into my arms. I held tight to her and quickly ushered her off the street into my home. I was much more frightened than she was, and alarmed that the violence might have been directed at her.

A few days later a social worker arrived and filled me in. Sabrina’s Moroccan mother had died, and her Algerian father was an alcoholic of no fixed abode. It was another relative who had flung the TV out of the window. The social worker explained that the family was chronically late with the rent and due to be evicted. However, in court at the eviction proceedings, a family member had testified that the child was so attached to me that to remove Sabrina from my vicinity would cause her permanent psychological damage.

“Is this true?” the social worker asked gingerly. I was completely stunned. This was a degree of manipulation I had never been subjected to before. Sabrina’s family was exploiting a casual friendship between me and this neglected little girl in order to keep their grip on a cheap flat. I swallowed hard.

“What can I do?”

“Well, if you go to the housing committee and tell them it’s not true, that there is no bond between you, the family will be evicted and the problem will be resolved. But…”

“But I won’t see Sabrina again,” I finished her sentence.

She nodded. “She will go into state care. The family has been judged not suitable to raise her, but they will have visiting rights.”

“What are the alternatives?” I asked.

“Well, you could tell me that it’s true, that there is a bond. Then perhaps we would ask you to consider legally fostering Sabrina. If the judge allows it.”

‘“You’re ruining your life,” I was told’: early days with former boyfriend Eric Adjani, his mother and Sabrina. Photograph: Lisa Lovatt-Smith

It took me a few minutes, but I began to realise that this bond conjured up by the family just might be real. I didn’t want Sabrina to end up in care. As a child I had been fostered for a year, so I knew how loved and secure it can make you feel. And I knew I could not abandon Sabrina.

Once I’d made that decision, it all moved very fast. By August, aged 23, I had become Sabrina’s official foster mother and she had moved in with me full time. My boyfriend, Eric, was not exactly approving, but accepting. He was a man who had faced many unusual situations in his life and he was of the opinion that since life was so hard, it was good to be as kind as possible. But he was straightforward with me: this kid was my idea, and I was to take full responsibility.

My fashionable friends were not as understanding. They didn’t think taking on Sabrina was a good idea. Many of them, particularly my editor-in-chief at the time, argued strongly against it. “You’re ruining your life,” I was told. They joked that no one would ever marry me now; not with a skinny dark-skinned child with a horrific background in tow.

My mum, who should have been the first person to get it, didn’t accept it either. My father had run away when I was four; disorientated and impoverished and determined not to go on to welfare, my mum had left me with friends for a year while she sorted her life out. She visited nearly every day, and the arrangement worked out beautifully. An affluent couple, Paul and Barbara, offered me safety and comfort at a time when my mum could not. It was the way temporary foster care is supposed to work, really. I felt that I could do the same for Sabrina. I felt I should pay it forward.

So it was confusing when my mum hardly spoke to me for a year, and refused to have much to do with Sabrina until she was almost 10 (she did, later, teach her to swim). Mum felt it was very limiting for me to saddle myself with a totally dependent child; she must have remembered how hard things were for her when I was about Sabrina’s age. What was most hurtful was that nobody seemed to think that I was actually doing something good for the child. I had no encouragement at all. International adoption was not as popular then as it is now. It was still very marginal, a deeply unusual choice, and the very words “adoption” and “fostering” (accueil) carried a burden of stigma and shame, especially in France.

Sabrina, however, took to her new life with flair. She was a sunny child and curious, and full of life – very skinny and with eyes way too big for her pretty face. The first problem I had to deal with were her rotten teeth, eaten away by a bad diet and not brushing. We had about eight visits to the dentist, and various doctors. At home I immediately, instinctively became very big on ritual. Cooking, playing, eating fruit, teeth-brushing, bedtime story… Sabrina had not had enough regularity and safety in her life and the everyday domesticity seemed to calm her. I had to cut back on my social commitments and the travelling for my job, but it was an obvious choice for me. If I was to make a go of this extraordinary adventure, Sabrina’s needs had to come first.

I did not have a TV, or any intention of acquiring one, so Sabrina grew up reading practically a book a day instead. Brioche was immediately annexed and became her exclusive property. That first summer we took a long TGV journey to a house in the south of France, and I remember clearly not knowing quite what to do with such an energetic child in such a confined space. But after that there were long walks with Eric and summer fêtes and cooking together and swimming with my mum.

‘I focused on being a mum, which I adored. It seemed I’d found my vocation’: at a charity gala in New York, with Sabrina. Photograph: Joe Schildhorn/BFAnyc.com

By September, when Sabrina walked away from me into her new classroom, joining the serpent of blue-and-white-clad six-year-olds on the their first day at “real” school, and the big iron gate closed behind her, I felt like bawling – just like any mother, but the rush of sentiment, took me by surprise. The depth of our feeling for one another was a revelation to me.

So now we were three. Eric’s sister Isabelle loaned us her huge, rambling country farmhouse in Normandy and we spent our weekends and holidays there. But when Sabrina was eight, Eric and I split up. He still did not have a job, and was dosing up on over-the-counter drugs which made him unresponsive and dull. Eric was always kind and loving to me, but he was absent, too, and I could not live with that. A few months afterwards he attempted suicide, and I was glad Sabrina was not exposed to that situation, although I’ve missed Eric everyday since; our easy domestic life was something I have never quite recaptured. He died two years ago.

I focused on being a mum, which I adored. It seemed that I had found my vocation. I told Vogue I was no longer available for international assignments, and I focused on packing picnics, singing songs and reading stories. I was fascinated with anything Sabrina liked. I saw Beauty and the Beast seven times; I made pancakes; I decorated her room so it looked like a ship; I went swimming with her in the local pool. In the summers we had endless sorties to the parks, and I helped her with her homework (a lot, as Sabrina was struggling with school). I shepherded her to the Louvre for the weekly children’s art class.

The challenges came mostly from her acting out at school. Sabrina’s excellent behaviour at home was not mirrored by what she did at school. It started with her forging my signature on homework that had earned bad marks, then she began to hide or destroy her books. She was very bright, had picked up English in a few months and was perfectly bilingual, but was naughty and trying for her teachers. Beneath all the chatter, she was secretive. I found out later that bad things were happening when she visited her family, and she was hiding them from me as she was instructed to by them: kicked-in doors, police visits, arrests and complaints by the neighbours. Heavy secrets for a little girl to hide.

My plan had always been to be a temporary foster mother, with Sabrina keeping strong ties to her birth family. I had no intention of separating her from her heritage. But her family in Paris were intrusive, demanding and manipulative. Despite their visiting rights they were constantly late or did not turn up at all – and yet continuously made the threat of demanding custody. Sabrina had a guardian, an older man who had known her birth parents, and he entreated me to protect her from the rough environment they lived in. Every few months he would turn up with a gift for Sabrina, and hiss: “Those siblings of hers are good for nothings. Mind that she never goes to live with them, eh? Or she will turn out just like them.”

I was in over my head. My fostering arrangement came up for review by a stern French judge every year. I dreamed of taking Sabrina away, where this tussle for custody could not reach her. But that would have meant defying the judge and breaking the law. So I kept quiet. I could not risk rocking the boat. My thoughts turned to the rest of her extended family. I wanted her to be familiar with her ethnic origins, and I did not want her to solely identify with her unhappy half-siblings. I knew that the Moroccan culture was rich and varied and I wanted her to learn to love that part of herself, too. Soon we were spending every holiday together in Morocco, and I began to learn Arabic. I loved the beauty of the country – the deep ochre of Marrakesh, the red mountains, the fresh green of the spring – and I felt deeply at home there.

Eventually we tracked down Sabrina’s extended birth family and visited them several times over the years. That was a joyous moment: ancient grandpa, tiny bird-like aunt, mysterious uncle who was a political dissident and a cheerful gaggle of assorted pre-teen cousins, all so happy to see her. I was asked to eat in the lounge with the men (the women ate in the kitchen) “because what you have done taking on this little girl is worthy of a man”. While the feminist in me balked, the mother in me swelled with pride.

The years since haven’t been easy but today, Sabrina is studying a masters degree in Paris, and has two adorable children of her own. Brave. Determined. That’s my girl.

Who Knows Tomorrow: A Memoir of Finding Family Among the Lost Children of Africa by Lisa Lovatt-Smith is published by Weinstein Books, £17.99