Soon after moving to London, Cossey started transitioning, after befriending a post-operative transsexual woman. By the age of 17, Cossey had started hormone therapy, was living full-time in a female gender role, and had begun a career as a showgirl at a London nightclub. Despite initial shock, her parents were eventually supportive. After undergoing breast augmentation surgery, she worked as a showgirl in Paris and as a topless dancer in Rome to save up for sex reassignment surgery (SRS). After years of hormonal and psychological treatment, and legally changing her name, Cossey had SRS on December 31, 1974 at Charing Cross Hospital in London.
Cossey won a part on the British game show 3-2-1 in 1978. Soon, a tabloid journalist started contacting Cossey, revealing he had discovered she was transsexual, and planned to write a story on it. Journalists began researching her past, and attempted to interview her family members. Cossey responded by dropping out of the show, convincing the producers to let her out of contract; eventually, the tabloid's journalists stopped contacting Cossey and her family. After the incident, Cossey tried to keep a lower profile, and accepted only smaller assignments.
In 1981, Cossey was cast as an extra in the James Bond film For Your Eyes Only. Shortly after the film’s release in 1982, the tabloid News of the World finally revealed that Cossey was transsexual, with a front page headline reading "James Bond Girl Was a Boy." The story spread quickly, and by her own accounts, Cossey was so upset by the press coverage she contemplated suicide. However, she was eventually able to continue her modeling career. In 1982, Cossey responded by releasing I Am a Woman, her first autobiography.
In 1985, after breaking up with Lasinio, she met Elias Fattal, a Jewish businessman, who was not aware of her history until proposing marriage on Valentine's Day 1988. Rather than rejecting her, as she had feared, he merely asked if she would convert to Judaism, which she did. They were married on May 21, 1989, a few weeks after the European Court of Human Rights handed down their decision to legally recognize Cossey as a female on May 9. They returned from their honeymoon to find that The News of the World had done another story on their wedding. Fattal's family was angry and horrified and after a few weeks, convinced him to have the marriage annulled.
On September 27, 1990, the European Court overturned their decision after the British government appealed. (Transsexuals born in the United Kingdom would later be declared legally female through the Gender Recognition Act 2004.) Cossey returned to modeling, which she had not done for the four years with Fattal.
In 1991, Cossey released My Story, her second autobiography which told the details of her transition and her unsuccessful battle with the European Commission. Cossey was featured in the September 1991 issue of Playboy, in pictorial "The Transformation Of Tula", this time as an acknowledged transsexual.