Mahoney played 21 games for the Bluejays after transferring from Southeast Missouri and averaged 12 points and 3.1 rebounds as a reserve wing.
He announced his decision on Thursday.
Mahoney can return to Creighton after going through the evaluation process by league executives as long as he doesn't sign with an agent and withdraws from the draft by June 3.
Alexander, a first-team All-Big East selection, declared for the draft last Friday.
Creighton finished 24-7, won a share of the Big East regular-season championship and was No. 7 in the final Associated Press poll.