Mulumbu, 26, put pen to paper on a deal on Tuesday that will keep him at The Hawthorns until 2016.
The DR Congo international has spent four years with West Brom and has been impressed by the continual progress the club made since joining from Paris Saint-Germain.
"I want to play in the best team I can, and I think at the moment West Brom match with that," he told the club's official website.
"Every year we just improve, we've got some decent players at this club like (James) Morrison, Jonas (Olsson) and (Chris) Brunt and if you add some great players like Nicolas (Anelka), I'm looking forward to the start of the season."
He added: "We're under pressure because at the moment we're the best team in the West Midlands, so we need to show it again this year, and I think we're going to respond on the pitch."
