The Championship club announced on Friday that James Webster - who had helped them win the 1895 Cup at Wembley and reach the Million Pound Game against Toulouse Olympique - had left his role.
McDermott, 51, has not held a head coach role since Toronto Wolfpack's demise last summer and had been working in an advisory capacity with Oldham after joining them in July.
The former Bradford Bulls prop won four Grand Finals as Leeds chief and will now be tasked with trying to get Featherstone into Super League for the first time.
They have lost both of the last two promotion deciders, the other ironically being against McDermott's Toronto in 2019.
"There is a decent project here," he told this morning's press conference alongside chairman Mark Campbell, CEO Martin Vickers and Rovers second-row Brett Ferres, who won one of those Super League titles with him in 2017.
"I understand the task ahead and I understand the club as well.
"When I was with the Rhinos that's where the origin of the dual-reg came in.
"I had many meetings with Mark Campbell about how dual-reg would work and to make dual-reg work I had to have an a understanding of the club, understand the philosophies and understand where Mark's at.
"The guy has got a burning ambition to get promoted. That's always been the case; I've always known that.
"Over the years Fev has grown: it's filled out, it's packed out and it's one of the most progressive clubs in the game.
"But it has got that single objective of getting promoted to Super League and that's what Mark brought me here for.
"I'm really excited. There are not many projects in the game. There are not many clubs that have the ambition that Featherstone Rovers have.
"But the script that lies before us of doing something that hasn't been done before with Fev is one of the main reasons I'm here."
Campbell said: “I’ve made it really clear in my tenure at the club that I’m committed to giving everything I possibly can to bring Super League status to this club, it is what this club this deserves.
“I’ve known and worked with Brian previously and I know he is the right fit for this club to take us to the next level.
"We have moved quickly to take this opportunity; it is not every day you get the chance to bring a top class coach like Brian to your Club and we are extremely excited about working together moving forward.”