St Giles' was founded in the 1120s when the Scottish royal family, the sons of Queen (Saint) Margaret and King Malcolm Canmore, especially David I (1124-1153) made strenuous efforts to spread Catholic Christian worship throughout the Scottish lowlands.
This church was probably quite small, Norman (i.e. Romanesque, with rounded arches and elaborate carving) in style, like others built at the same time. Few traces of it survive in the present building.
In 1385, a much larger church (early Gothic, pointed arches and simple octagonal pillars) was partially burned. No record has been found of the building of this second church. It was quickly repaired.
Over the next 150 years many chapels were added. These included chapels set up by the craftsmen's guilds of Edinburgh, chapels endowed by prominent merchants and nobles, and a chapel for a relic of St Giles. By the middle of the 16th century, there were around fifty altars in the church.