Provided is a method of manufacturing a liquid ejection head, which is capable of patterning a dry film while suppressing deformation of the dry film caused by a pressure. The method of manufacturing a liquid ejection head includes: preparing a substrate including an ejection orifice member on a first surface; forming, on an ejection orifice surface of the ejection orifice member, a protection film having communicating holes for allowing ejection orifices to communicate to outside; closing an opening of a supply port on a second surface on a side opposite to the first surface of the substrate with a dry film; and patterning the dry film by irradiating the dry film with light under a state in which the protection film is formed on the ejection orifice surface.