There, he met John Petrucci and John Myung. They formed a band, initially taking name "Majesty" before changing the name to Dream Theater. They left Berklee soon after.
In the April 2001 edition of Modern Drummer, Portnoy names Billy Cobham, Stewart Copeland, Jon Fishman, Dave Lombardo, Andy Sturmer, Chester Thompson, Nick D'Virgilio, and Alan White as influences.
Portnoy has won 23 awards from Modern Drummer magazine, including "Best Up & Coming Talent" (once), "Best Clinician" (two times), "Best Educational Video/DVD" (two times), "Best recorded performance" (6 times) and "Best Progressive Rock Drummer" (for 12 consecutive years), and, at 37, was the second youngest person (behind Neil Peart, at 31) inducted into the Rock Drummer Hall of Fame.
He has co-founded several progressive rock groups and projects, including Transatlantic, Liquid Tension Experiment, and O.S.I. (with former Dream Theater keyboardist Kevin Moore).
He has released three instructional videos, "Progressive Drum Concepts", "Liquid Drum Theater", which has won awards from Modern Drummer Magazine, and his latest "In Constant Motion". He has released many "Official Bootlegs" on his website, including footage of the studio sessions for the Dream Theater albums Six Degrees of Inner Turbulence, Train of Thought, Octavarium and the Transatlantic release Bridge Across Forever. He has released bootlegs of his tribute bands Hammer of the Gods (a Led Zeppelin tribute), Yellow Matter Custard (The Beatles), Cygnus and the Sea Monsters (Rush), and Amazing Journey (The Who).
On September 18, 2006 within his website's forum Portnoy cagily stated that he had finally met one of his drummer heroes from youth from the band Rush; drummer Neil Peart (one of the few heroes from his youth Portnoy had never met, owing to Peart's legendary privacy), that "Neil and I have finally met and we spent some time together this week and had a great time..." , and that a possible collaboration was underway/had been discussed with the teasing statement of "Patience everybody... all will soon be revealed!" It has now been revealed that this meeting was for an interview Portnoy did for his stint as guest editor of Rhythm Magazine. Portnoy had previously written the liner notes for the Japanese pressing of Rush's 2002 release Vapor Trails, but publicly denounced Peart in 2000 after Peart called the people involved in the Rush tribute "bar bands".
In the October 2009 issue of Rhythm Magazine, Portnoy was listed as #5 in their list of the Top 50 Drummers of All Time.
He was featured on Fuse TV's Talking Metal where he joined with Bobby Ellsworth and the show's hosts in covering Motörhead's "Overkill."
On February 17, 2010, Avenged Sevenfold announced that Portnoy will be filling in for their former drummer Jimmy "The Rev" Sullivan, who passed away in December. Portnoy was known to be Sullivan's all time favorite drummer, and has been quoted saying, "It would be an honor and, without question, that’s what Jim would have wanted." Portnoy will join Avenged Sevenfold in the studio over the next few months in order to record their upcoming album, which will be in Sullivan's memory. Speculation from many Dream Theater fans was that Portnoy would leave the band for the younger and darker Avenged Sevenfold.
As voted on by 6,500 drummers worldwide, Portnoy won the Drummies Award for Best Progressive Rock drummer in 2010, while Joey Jordison of Slipknot/Rob Zombie/Murderdolls won the award for Best Metal drummer.
Several of Portnoy's drum kits make use of Roland electronic drum pads. Portnoy uses these as what he calls the "secret cowbell", to count off the songs in a way that only his fellow band members and the technicians can hear it. An example of this can be seen on the Score DVD, where it can be seen in the background that Portnoy counts off the song "Under A Glass Moon" on the pad. Portnoy explains the use of these pads in the commentary track of the "Drum Cam Only" version of the Live At Budokan DVD, as well as the "Mike's Stage Tour" featurette on the Chaos in Motion 2007–2008 DVD.
Tama has marketed two different sized Mike Portnoy signature snare drums, known as "Melody Masters" (named after Melody, his daughter), since the late 1990s. The smaller of the two has a 12x5" steel shell and the larger has a 14x5.5" maple shell. Both snare models have a unique three way snare strainer mechanism.. Portnoy and Tama have custom-made versions in various colors to match the color schemes of the kit he was using at the time (i.e. The "Purple/Red/Green Monsters", "Siamese Monster" "Albino Monster", etc.). For 2006, in recognition of his 'Albino Monster' kit, he released only 250 of these snares worldwide in a white, gloss finish. The older colour (black) is not limited edition.
Sabian has marketed three different sized signature cymbal stacks, known as "Max Stax" (named after his son, Max). They consist of a hand hammered china kang-style cymbal on top of a splash (or crash) cymbal that can be played much more like a drum than a cymbal, used much more in traditional drum fills than most quickly decaying cymbal tones. Their sizes are 8/8", 10/10" and 12/14". Sabian has marketed odd-sized 7, 9, and 11" "Max Splash" cymbals which have a fast decay and can be used in fills and drum patterns that have the same namesake.
Portnoy has endorsed Promark sticks since the early 1990s. His signature stick is the TX420N . It is made of hickory 5A sized stick, with a white nylon 747 sized taper and has purple print to match the Purple Monster's motif. The "420" in the model number, which is printed on the sticks, is a reference to his birthday (and not a marijuana reference, as has been suggested). He is known for gripping his left stick on the reverse side to attain a different sound on the snare drum.
Portnoy is a member of tribute band called HAIL! HAIL!'s rotating cast of members include Andreas Kisser, Tim "Ripper" Owens, Paul Bostaph, David Ellefson, Jimmy DeGrasso and Roy Mayorga. Owens, DeGrasso, Ellefson and Kisser formed the band in late 2008. The band, toured Europe in 2009 and in June 2010 HAIL! is on their second European tour with the following line-up: Andreas Kisser, Tim "Ripper" Owens, Paul Bostaph and James LoMenzo. Due to the tragic passing of previously announced bassist Paul Gray, LoMenzo agreed on only one day’s notice to fly to Portugal and rescue the Rock In Rio Festival where HAIL! was scheduled to perform on May 30, 2010.
In spring 2010, Portnoy filled in for Avenged Sevenfold's former drummer, Jimmy "The Rev" Sullivan, who died during production of the band's latest album Nightmare.
On the May 5th, 2010, Portnoy released a statement on his status with Avenged Sevenfold on their official website saying: "Although I wish I could stay on board with A7X in a more permanent capacity, the reality is that I am committed to Dream Theater as my main priority and I will need to resume work with DT to start a new album at some point in 2011.....However, I will be able to join my brothers in Avenged Sevenfold for at least the duration of their touring throughout 2010.....You can look at me like the band's "rebound drummer" to buy them some time until they are comfortable and ready to make a commitment to somebody new for full-time relationship in their future!"
Since then, Mike Portnoy has revealed he will stay with Avenged Sevenfold throughout 2011 and the full album cycle (As revealed in a "Sabian" video during the Rockstar Uproar Tour).
On Wednesday, September 8, 2010, Mike Portnoy announced he would be leaving Dream Theater.
In his official statement he said: "Dream Theater was always my baby...and I nurtured that baby every single day and waking moment of my life since 1985...24/7, 365...never taking time off from DT's never-ending responsibilites (even when the band was "off" between cycles)...working overtime and way beyond the call of duty that most sane people ever would do for a band....But I've come to the conclusion that the DT machine was starting to burn me out...and I really needed a break from the band in order to save my relationship with the other members and keep my DT spirit hungry and inspired.
We have been on an endless write/record/tour cycle for almost 20 years now (of which I have overseen EVERY aspect without a break) and while a few months apart from each other here & there over the years has been much needed and helpful, I honestly hoped the band could simply agree with me to taking a bit of a "hiatus" to recharge our batteries and "save me from ourselves"...
Sadly, in discussing this with the guys, they determined they do not share my feelings and have decided to continue without me rather than take a breather...I even offered to do some occasional work throughout 2011 against my initial wishes, but it was not to be..."
The rest of the band did not agree and decided to continue on without him. Because of this decision, Portnoy decided he will be leaving Dream Theater. How the production of the 11th album or the future world tour will continue on without him is still unclear. According to John Petrucci's Twitter account, auditions to find a new drummer will begin in October. An interview with James LaBrie conducted on October 8, 2010 confirmed that there were seven world-class drummers lined up for auditions, which would be taking place through the rest of October. James is confident that their new drummer lies within these seven.
Portnoy and his wife Marlene (née Apuzzo) live in the Lehigh Valley with their children, Melody Ruthandrea and Max John. Marlene used to be a guitarist in the all-female band Meanstreak, and two of the other members, guitarist Rena Sands and bassist Lisa Martens, are married to Dream Theater guitarist John Petrucci and bassist John Myung, respectively.
Portnoy identifies as a recovering alcoholic and substance abuser and is an active member of Alcoholics Anonymous. Dream Theater's Twelve-step Suite deals with his struggles.