He graduated from Cardiff Medical School in 1971 with a bachelor's degree (MB,BCh) and in 1973 was awarded a diploma by the Royal College of Obstetrics & Gynaecology.

[1][2] After moving to Swansea, Evans was appointed as a full-time clinical consultant paediatrician at Singleton Hospital in 1980 and became involved in the development of newborn intensive care and maternity services.

[3] In 1990, the Swansea Bay Health Board built a new children's department and neonatal unit that Evans helped to design; between 1992–1997 and 2004–2008 he then served as clinical director of paediatrics and neonatology.

In 1990, speaking in his capacity as West Glamorgan County councillor, Evans is reported to support an argument to decriminalise sexual offences against children.

[13][14] He was an expert witness in the case of an inquest into the death of a four-year-old girl from salt poisoning; Evans identified broken ribs as a comorbity suggestive of potential abuse.

Justice O'Hara stated in his reasoning that:In his judgment the trial judge was critical of Dr Evans, suggesting that his evidence should have been more considered and structured than in fact it was.

That is especially so in cases such as the present and lawyers should be far more alert to avoid drafting letters of instruction to doctors inviting them to express their opinion on that issue.

[22]Evans was an expert witness in the 2021 murder trial of a man later convicted of killing his 5 month old daughter,[23] being able to rule out pneumonia as a potential cause of death in favour of a blunt force brain injury.

It ends with tendentious and partisan expressions of opinion that are outside Dr Evans' professional competence and have no place in a reputable expert report.

"[25][1] During the police investigation which followed, Evans was instructed to review clinical records of the babies in the unit who had died or collapsed suddenly, in total 61 cases.

[42][43] After the trial, upon reviewing the descriptions of the infants' skin discolourations, Dr Shoo Lee, author of the 1989 paper used by the prosecution, disagreed that they are suggestive of air embolism.

[46][1][27] Following Letby's failed appeal, Dr Lee and a group of 14 international neonatology experts conducted a private review of the deaths.

[49] The Royal Statistical Society criticised the prosecution's evidence, citing similaries to the medical miscarriage of justice cases of Lucia de Berk and Daniela Poggiali.

[50] Evans was in turn critical of The Royal Statistical Society's suggestions, as well as the Cheshire Coroner and other expert neonatologists during his November 2024 interview with Sweeney.

[28] During the November 2024 interview by John Sweeney, Evans acknowledged the known presence of pseudomonas aeruginosa in the neonatal unit's water supplies, leading to several cases of pneumonia.

[51][52] Speaking in Hansard in January 2025, Sir David Davis MP, who chaired the February 2025 Dr Lee panel had already lead a debate in the House of Commons on Letby's trial, stating: "It is a clear miscarriage of justice by a judicial system that could not manage admittedly difficult statistical and medical scientific evidence.

Davis was then quoted in Hansard as saying: "It is in essence saying, 'If your defence team weren't good enough to present this evidence, hard luck, you stay banged up for life.'