Within Styx, DeYoung acted as lead vocalist, keyboardist, accordion player, producer, writer and creative force behind many of the band's hit songs. A self-taught keyboardist, DeYoung quickly became one of the most notable players of that instrument in rock. Featured on the cover of the January 1981 issue of Contemporary Keyboard magazine (a story that was reprinted in Contemporary Keyboard's book on the greatest rock keyboardists), DeYoung described many of his steps along the way through his keyboard-playing career: He'd never played an acoustic piano until the recording session for 1972's "Lady"; he recorded the track for 1979's "Babe" in a friend's basement on a Rhodes electric piano he'd never touched before; the odd feeling of switching back to playing accordion for the song "Boat On The River" and discovering how small the keys felt to his fingers after years of playing electric organs and pianos.
As a keyboardist in Styx, DeYoung was best remembered for his prominent lead synthesizer solos performed on the Oberheim synthesizer that dominated the mix with a unique tone, a key element of the Styx sound. DeYoung pioneered the use of synthesizers in rock and roll. Influenced by the recent release of Emerson, Lake & Palmer's first album, DeYoung — a novice synthesizer player at the time — used a modular Moog to record the keyboard tracks for the first Styx album. This album featured a rock version of "Fanfare for the Common Man", more than 5 years before ELP came up with a similar idea of recording this classical composition as a rock band featuring the synthesizer that would later become one of ELP's best known recordings.
DeYoung's songs often had a grandiose style to them in the tradition of 1970s theatrical rock, which heavily influenced the group's direction in the late 1970s, culminating in the concept albums Paradise Theatre (1981) and Kilroy Was Here (1983). The dissent of some members in the band during Kilroy brought tensions between the group's members over the future direction of the band, leading to guitarist Tommy Shaw's departure in 1984.