Introduction Optimum mammography positioning technique is necessary to maximise cancer detection. Current criteria for mammography appraisal lack reliability and validity with a need to develop a more objective system. We aimed to establish current international practice in assessing image quality (IQ), of screening mammograms then develop and validate a reproducible assessment tool. Methods A questionnaire sent to centres in countries undertaking population screening identified practice, participants for an expert panel (EP) of radiologists/radiographers and a testing panel (TP) of radiographers. The EP developed category criteria and descriptors using a modified Delphi process to agree definitions. The EP scored 12 screening mammograms to test agreement then a main set of 178 cases. Weighted scores were derived for each descriptor enabling calculation of numerical parameters for each new category. The TP then scored the main set. Statistical analysis included ANOVA, t-tests and Kendall's coefficient. Results 11 centres in 8 countries responded forming an EP of 7 members and TP of 44 members. The EP showed moderate agreement when the scoring the mini test set W = 0.50 p < 0.001 and the main set W = 0.55 p < 0.001, ‘posterior nipple line’ being the most difficult descriptor. The weighted total scores differentiated the 4 new categories Perfect, Good, Adequate and Inadequate (p < 0.001). Conclusion We have developed an assessment tool by Delphi consensus and weighted consensus criteria. We have successfully tabulated a range of numerical scores for each new category providing the first validated and reproducible mammography IQ scoring system.