Porcaro wrote or co-wrote at least one song on each of Toto's first six albums, with the exception of Isolation. Unlike most of his bandmates, he generally did not contribute lead or even backing vocals; he considered himself a weak singer, and sang lead on two of his earlier compositions only because he felt that the vocal styles of his bandmates, with the exception of Joseph Williams, were not appropriate for his songs. He left Toto in 1986 after the Fahrenheit album in order to pursue a more full-time songwriting and composing career. Porcaro composed the music for the song "Human Nature" and produced the synthesizer for "The Girl Is Mine" from Michael Jackson's album Thriller, and also played with Gary Wright during his tour in 1977, supporting the latter's The Dream Weaver album.
His two older brothers Mike and Jeff were Toto's bass player and drummer, while his father Joe is a prolific session percussionist.
Steve Porcaro is currently working on film scores. He collaborated with his daughter Heather Porcaro on her debut album, The Heartstring Symphony, released in 2009. He composes the music for the FX television show Justified.
He also returned to play with Toto at live performances since 2010, when they decided to reform the band and tour in Europe to support Mike Porcaro. They have scheduled some more concerts in Europe. Porcaro performed on the band's 2015 studio album Toto XIV, co-writing and singing lead on 'The Little Things'. This was only the third lead vocal performance from him on a Toto album.
Steve Porcaro played a huge variety of synthesizers on Toto albums. While David Paich handled piano duties, it was usually Steve's job to complement the music with creative synth sounds. He was known to use Yamaha GS1, Yamaha DX1, Yamaha CS80, Roland Jupiter-8, Minimoog, Polyfusion Modular and Sequential Circuits Prophet synthesizers among others. The synth solo in the Toto song Rosanna is a noted example of his approach. 
Nowadays he prefers working with software synthesizers. Onstage he is using two Yamaha Motifs to drive his Apple Mainstage based virtual synth rig.