He left Inner Sanctum (his last high school band) after being awarded a scholarship to attend the Berklee College of Music in Boston.
In university, he met John Petrucci and John Myung.
They formed a band, initially taking the name "Majesty" before changing their name to Dream Theater.
They left Berklee soon after.

Known for his technical skill as a drummer in Dream Theater, Portnoy has won 30 awards from the Modern Drummer magazine.
He co-produced six Dream Theater albums with guitarist John Petrucci, starting from Metropolis Pt.
2: Scenes from a Memory through Black Clouds and Silver Linings and was one of the main songwriters during his tenure.
He is the second youngest person (after Neil Peart) to be inducted into the Modern Drummer Hall of Fame, at 37 years of age.

In the spring of 2010, Portnoy filled in for Avenged Sevenfold's former drummer, Jimmy "The Rev" Sullivan, who died during the production of the band's fifth album Nightmare.
On May 5, 2010, Portnoy released a statement on his status with Avenged Sevenfold on their official website, concluding his working relationship with the band.
On September 8, 2010, Mike Portnoy announced he would be leaving Dream Theater.
The band hired Mike Mangini to take his place after inviting and holding auditions with seven "world-class" drummers.
 When Avenged Sevenfold later decided to also sever ties with Portnoy, he announced that he had asked to return to Dream Theater, but was turned down by their lawyer since the band had already brought Mangini on board, and Mangini had left his tenured teaching position at Berklee College of Music to join the band.
He left Inner Sanctum (his last high school band) after being awarded a scholarship to attend the Berklee College of Music in Boston. In university, he met John Petrucci and John Myung. They formed a band, initially taking the name "Majesty" before changing their name to Dream Theater. They left Berklee soon after.
Known for his technical skill as a drummer in Dream Theater, Portnoy has won 30 awards from the Modern Drummer magazine. He co-produced six Dream Theater albums with guitarist John Petrucci, starting from Metropolis Pt. 2: Scenes from a Memory through Black Clouds and Silver Linings and was one of the main songwriters during his tenure. He is the second youngest person (after Neil Peart) to be inducted into the Modern Drummer Hall of Fame, at 37 years of age.
In the spring of 2010, Portnoy filled in for Avenged Sevenfold's former drummer, Jimmy "The Rev" Sullivan, who died during the production of the band's fifth album Nightmare. On May 5, 2010, Portnoy released a statement on his status with Avenged Sevenfold on their official website, concluding his working relationship with the band. On September 8, 2010, Mike Portnoy announced he would be leaving Dream Theater. The band hired Mike Mangini to take his place after inviting and holding auditions with seven "world-class" drummers.  When Avenged Sevenfold later decided to also sever ties with Portnoy, he announced that he had asked to return to Dream Theater, but was turned down by their lawyer since the band had already brought Mangini on board, and Mangini had left his tenured teaching position at Berklee College of Music to join the band.
Mike Portnoy