Miller has pectus excavatum, a deformity of the thoracic wall which causes the chest to cave in due to the sternum and rib cage growing abnormally.

[2] Miller opted not to have surgery to correct the deformity, and swimming helped broaden his chest and develop his rib cage.

[11] Miller rose to prominence in the world swimming scene at the 2014 Short Course World Championships in Doha, Qatar, where he took home a pair of medals, a silver in the men's 4×100 meter medley relay (3:21.49) and a bronze in the 4×50 m medley relay (an American-record time of 1:31.83).

At the 2015 World Aquatics Championships in Kazan, Russia, Miller claimed his first gold medal as part of the U.S. team in the 4×100 meter medley relay.

Swimming the breaststroke leg, Miller recorded a split of 59.23 seconds in the prelims to put the Americans at the top of the final field with a time of 3:31.06.

[25][26] In preparation for the 2020 U.S Olympic Trials, with the primary meet set to be held from June 13–20, Miller signed with Speedo after being with TYR for over 6 years.

[27] Legend: b — b-final Cody Miller became engaged to Ali DeWitt on November 22, 2015, at the Golden Goggle Awards.

On June 21, 2020, Miller revealed in a video on his YouTube channel that he and his wife Ali were expecting their first child.