Agorwyd Sain Ffagan i'r cyhoedd am y tro cyntaf oll ar Orffennaf 1af 1948. Ar y pryd, y cyfan oedd i'w weld yno oedd Castell Sain Ffagan, y plasdy hardd o oes Elisabeth I, a'r gerddi a'r tiroedd o'i gwmpas - rhodd i'r genedl gan Iarll Plymouth. Ers hynny, tyfodd Sain Ffagan i fod yn un o amgueddfeydd awyr agored gorau Ewrop. Ailgodwyd dros 40 o adeiladau gwreiddiol o bob rhan o Gymru er mwyn dangos sut oedd pobl yn byw mewn gwahanol gyfnodau. Yn rhai o'r adeiladau, mae crefftwyr wrth eu gwaith — y pobydd, y gof, y turniwr coed, y crochenydd a'r gwehyddion. Yn aml mae modd prynu eu cynnyrch. Gellir gweld bridiau traddodiadol o anifeiliaid a ffowls yn y caeau, ac yn arbennig ym muarth ffermdy Llwyn yr Eos. Yno hefyd bydd arddangosiadau o goginio traddodiadol o bryd i'w gilydd. Mae'r orielau cynhwysfawr yn arddangos bywyd beunyddiol y werin bobl, gwisgoedd ac offer amaethyddol. Cynhelir amrywiaeth o arddangosfeydd arbennig a digwyddiadau'n cynnwys arferion, cerddoriaeth a dawnsio traddodiadol.

St Fagans is one of Europe's foremost open–air museums and Wales's most popular heritage attraction. It stands in the grounds of the magnificent St Fagans Castle, a late 16th-century manor house donated to the people of Wales by the Earl of Plymouth. During the last fifty years, over forty original buildings from different historical periods have been re-erected in the 100-acre parkland. The re-erected buildings include houses, a farm, a school, a chapel and a splendid Workmen's Institute. There are also workshops where craftsmen still demonstrate their traditional skills. Their produce is usually on sale. Native breeds of livestock can be seen in the fields and farmyards, and demonstrations of farming tasks take place daily. Visitors gain an insight into the rich heritage and culture of Wales, and the Welsh language can be heard in daily use amongst craftsmen and interpreters. There are also galleries with exhibitions of costume, daily life and farming implements. Special exhibitions are also held regularly. Throughout the year, St Fagans comes to life — literally — as traditional festivals, music and dance events are celebrated.