Marcia was the daughter of Quintus Marcius Rex but her mother is unknown. The date and place of her birth have not been found.
Her husband was Caius II Julius. They were married, but the date and place have not been found. Their three known children were Caius III (c0140BC-0085BC), Sextus III Julius Cæsar (?-0090/89BC) and Julia Cæsaris (c0130BC-0069BC).