[7] Cass spent her early medical career in a general practice training scheme, during which she changed to paediatrics.

[6] During that 15-year period she also held some regional and national positions in medical education and policy development, including head of the London School of Paediatrics.

[6][13] At GOSH, she authored a book titled Snakes and Ladders, based on a programme she led that used role play to understand patient journeys in the NHS and to teach staff clinical governance skills.

[16] She reported on a large study she led that found no difference in urinary opioid peptide levels between those with autism and control subjects, and concluded that "opioid peptides can neither serve as a biomedical marker for autism nor be employed to predict or monitor response to a casein- and gluten-free diet".

[22] It said that the rise in referrals had led to the staff being overwhelmed, and recommended the creation of a network of regional hubs to provide care and support to young people.

Cass hoped that "every young person who walks through the door should be included in some kind of proper research protocol" and for those "where there is a clear, clinical view" that the medical pathway is best will still receive that, and be followed up to eliminate the "black hole of not knowing what's best".

However, she also expressed concern that her review was being weaponized to suggest that trans people do not exist, saying "that's really disappointing to me that that happens, because that's absolutely not what we're saying."

She also clarified that her review was not about defining what trans means or rolling back health care, stating "There are young people who absolutely benefit from a medical pathway, and we need to make sure that those young people have access — under a research protocol, because we need to improve the research — but not assume that that's the right pathway for everyone.

"[30] In 2012 Cass was appointed president of the Royal College of Paediatrics and Child Health for a three-year tenure.

[1][11] Cass was appointed an Officer of the Order of the British Empire (OBE) in the 2015 New Year Honours for services to child health.