Bennett, from Victoria, underwent a battery of tests in recent weeks and is expected to begin radiation therapy in the coming days, Swimming Canada said Tuesday.
"It's been a month of working through the diagnosis, now we're actively seeking early treatments," national team physician Steve Keeler said in a release. "It's come on quite suddenly. People associate lung cancer with bad habits, but that's not the case at all. There's a small subset of people who develop lung cancer who aren't smokers.
"Randy has been diagnosed with a malignancy that's going to require in-hospital care for the foreseeable future. It will involve both radiotherapy and chemotherapy."
Bennett, 51, served as national team coach at the 2012 London Olympics and has coached Canada's top senior teams at every major international competition since 2009. He has been with the High Performance Centre-Victoria since 2008 and worked with two-time Olympic medallist Ryan Cochrane, 2013 world championship medallist Hilary Caldwell and 2012 Olympian Alec Page.
"I think it's important people realize I'm not lying in bed curled up in a ball," Bennett said. "I am OK. It is what it is. You deal with it. I'm not rolling over. My expectation is that I'm going to get through this."
Keeler has been acting as Bennett's primary physician of late, working alongside a team of specialists.
"I'm impressed with how he's managing this," Keeler said. "I sat with him for two hours Saturday night watching hockey, sharing some stories and lots of smiles. He's most concerned about family, friends and his athletes.
"If he responds well to the first round of treatment we anticipate he'll be back in his normal role in the swimming community as a parent and coach within the next couple of months."
Bennett began his coaching career in Fort McMurray, Alta., in 1982.