Reed shot a 9-under 63 during the opening round at the Arnold Palmer Private Course, one of three used for this event, and had a one-stroke advantage over four players.
This pro-am tournament is known producing low scores and Thursday's first round was no exception. Of the 154 players who completed the first round, 112 of them broke par.
Johnson has climbed to No. 6 in the world rankings following his win two weeks ago at the Tournament of Champions.
The Jack Nicklaus Private Course and the LaQuinta Country Club are included in the three-course rotation The Palmer Course will be used for the final round on Sunday.
Reed, whose initial tour win came last year at the Wyndham Championship, birdied five holes in a row through the middle portion of his round.