Marlowe is vacating the position to focus more time toward developing projects under his overall deal with ABC Studios, although he'll continue to have a day-to-day presence on the show.
"Over the past four seasons, David has proven himself to be a tremendous leader and a great steward of our show's unique voice," said Marlowe in a statement. "I'm excited to continue our creative collaboration as he assumes his new responsibilities."
Amann became an executive producer on "Castle" in 2010 at the beginning of Season 3.
Marlowe lets go of the shows reins as "Castle" is on top. It also saw a 3 percent bump over the previous season, making Season 6 its most-watched season ever. It also ranked No. 2 in its 10 p.m. time slot for total viewers and the advertiser-coveted adults 18-49 demographic. And it is ABC's No. 1 ranked drama for TV playback in total viewers, adding an additional 4.3 million viewers to its live audience.
Gersh represents Amann and CAA represents Marlowe.