Khashoggi, a Washington Post columnist, was last seen at the Saudi consulate in Istanbul on October 2, 2018, where he was to receive papers ahead of his wedding.
His body was reportedly dismembered and removed from the building, and his remains have not been found.
Cengiz said that while she was waiting outside for him she had wedding plans on her mind.
"At the time I thought I would become Jamal's wife.
Instead I became the last witness before a murder," she told Swedish broadcaster TV4 in an interview on Monday (September 30).
A global outcry ensued and led to U.S. Treasury sanctions on 17 Saudi individuals and a Senate resolution blaming Prince Mohammed bin Salman, of whom Khashoggi was a critic.
The Saudi crown prince told CBS program "60 Minutes" that as de facto Saudi leader he ultimately bore "full responsibility" for the killing a year ago, but he denied ordering it.
Cengiz said that many questions about the murder remained unanswered.