St Illtyd’s can trace its origins back to two predecessor schools.

St Illtyd’s College was founded in 1923, when the De La Salle Brothers came to Cardiff to open a Roman Catholic school for boys. The school was opened as a grammar school on a site in Courtenay Road in the Splott area of the city. The buildings were extended in 1932 and the Preparatory department was moved to a new additional site on Richmond Road. The main school building was badly damaged by a bomb in 1941 during the Second World War. Severe overcrowding due to the damage sustained and growing pupil numbers led to the school moving to its current location on Newport Road in Rumney. A house was built at the rear of the school for the La Sallian Brothers to live in. When the Brothers left the school, this building became the Pastoral Resources Centre for the Archdiocese of Cardiff.

Heathfield House was a Roman Catholic grammar school for girls, founded by the Sisters of Providence of the Institute of Charity in 1867. It was initially called St Joseph’s Grammar School, though it moved to Heathfield House on Richmond Road in 1877 which led to the school changing its name. In the 1970s the school moved again, to a site on Ty Gwyn Road in Penylan.

The two schools were merged as part of the reorganisation of Catholic education in 1987. The school was renamed St Illtyd’s Catholic High School, and was now a co-educational school for pupils aged 11-16. Sixth form provision was transferred to the new St David’s Catholic College situated in the vacated Heathfield campus on Ty Gwyn Road.

The school was rebuilt on its existing site in 2002-3. The Assembly Hall, PE block and Sports Hall were refurbished at the same time, whist the other parts of the old buildings were demolished.

Whilst the school is no longer formally a De La Salle school, it still maintains links with bodies that are, such as St Cassian’s Retreat Centre. The five pointed star of the De La Salle Brothers is still contained within the school’s logo.