Phillips began to play professionally at the age of twelve in his father's (Sid Phillips) Dixieland band for four years. He was then offered the chance to play in the musical Jesus Christ Superstar. He quickly became a sought-after session player and in 1992, he released two instructional videos displaying some of his best work.
He has released four studio albums of varying styles.
In 2006, Phillips released a DVD with his jazz band, Vantage Point, called Resolution with writer and pianist Jeff Babko, trumpeter Walt Fowler, saxophonist Brandon Fields and bassist Alphonso Johnson.
In 1992, Simon was asked to fill in for Jeff Porcaro after Porcaro died during rehearsals for the upcoming tour to promote Toto's Kingdom of Desire album. Steve Lukather stated that he would only go on tour so shortly after Porcaro died if Toto could get Simon to play the tour, so he was the first and only drummer that the band contacted to replace Porcaro. After the 1992/93 tour Simon was asked to join Toto as a permanent member.
After the decision (15 July 2008) "to go their separate ways and begin new chapters in their lives", on 27 February 2010 former members of Toto announced on official band's website a reunion for a brief 2010 summer tour in Europe to honor their brother Mike Porcaro who is living with ALS (Lou Gehrig's disease). They also have scheduled more dates for a tour in 2011, ended up with the DVD performance in Villafranca Di Verona. Phillips is going to be touring with Toto in 2012 and 2013. After the last attempts at releasing a new DVD were all shelved the band recorded another concert aimed to be released on DVD in Lodz, Poland.
As of January 26, 2014, Phillips has departed from Toto to focus on his solo career. He is replaced by Keith Carlock.
In 2009, Phillips joined with keyboardist Philippe Saisse and bassist Pino Palladino in forming an instrumental jazz/funk rock trio: Phillips Saisse Palladino or PSP, who toured in Europe in 2009 through 2010, spotlighting the talents of each performer in the songs chosen for their set list. Phillips also performed on Joe Satriani's album Super Colossal, appearing on multiple tracks. Phillips appears in Alan Parsons' Art & Science of Sound Recording educational video series, as well as the program's single "All Our Yesterdays". He played in the Michael Schenker Group album In the Midst of Beauty and took part to the band's 30th Anniversary world tour in 2010. Phillips is featured on Hiromi Uehara's 2011 album, Voice.