officer and civil servant are both correct, memur is any person working for civil service - branches of public service concerned with all governmental administrative functions outside the armed services.
where the word comes from, what it means in the language it is borrowed from doesn't always tell you much. Loan words can change meanings and often used only in a specific context. A civil servant is never an işçi. If you want detailed explanations of Turkish words, use a monolingual dictionary, http://www.tdk.gov.tr/index.php?option=com_bts
A civil servant does not necessarily hold a position of authority. http://www.merriam-webster.com/dictionary/official
Sometimes the use of the word official refers to an officer, but an official is not always an officer. So, if you had the words "government official" that could be similar.