'long-serving' originates from Modern English compounding of 'long' and the present participle 'serving' (from 'serve'), where 'long' ultimately comes from Old English 'lang' meaning 'long' and 'serve' comes via Old French from Latin 'servire' meaning 'to serve'.

'long' changed from Old English 'lang' into Middle English 'long'; 'serve' changed from Latin 'servire' to Old French 'servir' and Middle English 'serven/serve' and eventually became the modern English 'serve', and the compound 'long-serving' was formed in Modern English by combining these elements.

Initially the components meant 'long (in duration)' and 'to serve'; over time the compound came to mean specifically 'having served in a role or provided service for a long time' in modern usage.