Titmuss was born on 8 September 1976. She grew up in Ruskington, where her parents were teachers, and took her A levels at Kesteven and Sleaford High School. She played the clarinet at school. Her parents divorced when she was 17, and her father moved to Argyll, Scotland, where he re-married. She graduated from City University in St Bartholomew's Hospital in 1998, and became a staff nurse at London's University College Hospital. She also worked at the Royal Brompton Hospital, and while there she was harassed by a senior house officer, Dr. Shibley Rahman, in 2004. After these incidents (which included confronting Titmuss while she showered naked), Dr. Rahman was eventually found guilty of misconduct by a General Medical Council tribunal.
Titmuss met TV presenter John Leslie at a party in pub on Fulham Road in London in 1998. The pair quickly formed a relationship, and Titmuss lived between her student nurse accommodation at the hospital and Leslie's home in Barnes, south-west London.
In 2002 presenter Ulrika Jonsson released her autobiography, in which she suggested that she had been raped earlier in her career by a now well-known TV presenter. Weeks later, the News of the World ran front page pictures of John Leslie snorting cocaine in a cafe. The following week, pictures appeared in the paper of Leslie, Titmuss and other women taking part in sex-sessions at Leslie's home, including Lanre Onabowale, posing as basketball player Jayson Blayde, and his girlfriend.