Mason, 60, is a senior national correspondent for the network and the co-anchor of the Saturday edition of "CBS This Morning".
Pelley was dubbed "Poison Pelley" after he supposedly screamed at a reporter in 2013, 'You wouldn't last 10 seconds at CBS News!'
Employees saw movers cleaning out Pelley's desk on Tuesday, but that was reportedly at the request of the veteran broadcaster himself, who is now on assignment in the Middle East for 60 Minutes.
Pelley, 53, became the anchor in 2011, replacing Katie Couric.
Pelley said: "CBS has been great to me for almost 30 years". Thus far, the network has not commented on the news.
CBS News appeared to be caught flat-footed as the news leaked out, first reported by the New York Post.
The network announced Wednesday that Pelley will return to work full time at the network's flagship newsmagazine "60 Minutes" as it begins its 50th anniversary season. "I'm glad to accept this assignment with continuing gratitude".
But the bump could be attributed to the impact that reporting on the Trump administration has had on viewership and readership at many news organizations. Some inside CBS News say he could eventually land the job officially. He also frequently contributes to CBS Sunday Morning. But sources told The Post that the network decided not to renew Pelley's contract after unsatisfactory ratings during his tenure.
Pelley will anchor the show for an indefinite amount of time before making the move to "60 Minutes". The CBS Evening News is the nation's longest-running nightly television news program, but its anchor does not hold the same role in setting the nation's news agenda that predecessors had a generation or two ago.
Pelley hasn't anchored the newscast since Thursday.
Tuesday night, the magazine, Page 6 reports Pelley is being moved to a role exclusively with '60 Minutes'.
While CBS has a significant roster of talent on hand, an anchor replacement has not yet been named.