Dorion, 43, has been with the Senators for nine seasons, the past two as assistant general manager.
"The reason we are doing this today is we want our fans to know that our management is in place and as most of you are aware, Bryan has been the biggest influence in my professional and private life," Dorion said. "He has decided to step down. I'm so thankful and extremely humbled to be following in Bryan's footsteps. Having to learn from someone who will and should be in the Hall of Fame is something I can only be thankful for. Bryan has been a tremendous mentor. He has taught me what it takes to be an excellent GM … I am stepping into big shoes."
The Senators finished fifth in the Atlantic Division with a 38-35-9 record and missed the Stanley Cup Playoffs for the second time in three seasons. Dorion said no immediate decisions would be made on the future of coach Dave Cameron and his staff.
Dorion will have exit meetings with Cameron, his staff and players Monday.
"There's an evaluation process," Dorion said. "Sometimes we're quick to judge. There will be changes, but I think we have to do the proper changes and we have to take our time doing it, and make sure that every change we make is the right one.
"As far as the future, anyone who falls under the umbrella of hockey in this organization will be evaluated. We can all talk about accountability, but for me results bring accountability. We are in the business of winning and positive results. People who know me know that I'm driven, sensitive, passionate, demanding, but most importantly for me it's usually black or white.
"For us not making the playoffs was simply unacceptable. I wasn't hired here to bring the team to the playoffs, but to succeed in the playoffs."
In Dorion, the Senators have a man who continues in the family business. His father, the late Pierre Dorion Sr., was the head scout for the NHL's Central Scouting Bureau and the Toronto Maple Leafs.