'long-tenured' originates from English, specifically the words 'long' and 'tenured'. 'Long' ultimately comes from Old English 'lang' where 'lang' meant 'long'. 'Tenured' derives from 'tenure', which comes from Old French 'tenure' and ultimately Latin 'tenēre', where 'tenēre' meant 'to hold'.

'long' changed from Old English 'lang' into Middle and then modern English with little change in basic meaning. 'Tenure' came into English via Old French 'tenure' (from Latin 'tenēre'/'tenēre' → 'tenir' in Gallo-Romance), and the adjective 'tenured' was formed in modern English by adding the participial/adjectival -ed to 'tenure'. The compound 'long-tenured' is a modern English combination of 'long' + 'tenured'.

Initially the elements meant 'long' (duration) and 'to hold' (possession/holding); together as 'long-tenured' the term has come to mean specifically 'having held a position or job for a long time' with emphasis on length of service rather than legal tenure alone.