The two Lewis schools trace their history to the will of their founder, Edward Lewis, a wealthy aristocrat, who traced his ancestry to Gwaethod Fawr, Prince of Cardigan in the tenth century. Despite his privileged upbringing, Edward Lewis was mindful of the poverty and deprivation in the lives of ordinary people, and so, in his will of 1715, he made provision for the education of "fifteen poor boys". It is believed that by 1853 the foundation catered for the education of girls, but at an elementary level only.

In 1882, the Gelligaer School Board decided that a secondary school for girls was a necessity, and eventually, the School Board offered a room, the West Room, at their infants’ school at Pontlottyn. It was opened on 2nd February 1883 with Miss Emma Edwards as headmistress. Forty girls attended; twenty-one having free places.

Very soon the school became overcrowded and in 1896 a new building was handed over and became the Gelligaer Intermediate School for Girls.

The dawning of the 20th century witnessed enormous changes in the Fortunes of Lewis Girls' School. The new school building, known as Hengoed Girls' County School, was formally opened on 1st November 1900. A new Headmistress, Miss Winifred James was appointed at the same time. A sub-committee of the Governors had been given the task of designing the school grounds, which were also to include tennis courts.

County Alderman Tom Evans unveiled the Commemorative Tablet in the Entrance Hall, officially opening the new school in Oakfield Street, Ystrad Mynach. "Between 1100 and 1200 people attended this event, an historuc and memorable occasion" which was the beginning of Lewis Girls' Grammar School. Miss Green was appointed as Headmistress in 1959.

Lewis Girls' Comprehensive School was formed in 1973. Ystrad Mynach Secondary School, which was to become Lower School, merged with Lewis Girls' Grammar. Although the site at Oakfield Street was very modern, it had been designed to accommodate approximately five hundred Pupils. Suddenly the school had doubled in size!

We finally turned full circle when Block Y was opened and the whole school was reunited on one site. The new teaching block, known as Block Y, was officially opened onm Friday 14th January 1994 by our Chairman of Governors, County Councillor Leonard Lewis. The new block provided many extra facilities, including modern languages, science laboratories, Information Technology rooms, a typing room, a Busines Studies room, Welsh language rooms and a Special Needs classroom.

In 2007 a new block was built Nightingale Block. This provided expanded facilities for the English department, Media Studies and Business studies. Since then Lewis Girls' School has continually modernised and maintained it's buildings in order to meet the needs of its pupils.