Spokesman Howard Bragman said McMahon, 85, had been hospitalized for several weeks for treatment of pneumonia and other ailments. He declined to identify the Los Angeles facility.
"It's serious," Bragman said when asked about McMahon's condition, noting his age. But, Bragman added: "We're hopeful."
A person close to McMahon, speaking on condition of anonymity because he was not authorized to release the information, said the illnesses included bone cancer.
Bragman said McMahon's wife and family are with him and appreciate the public expressions of concern.
McMahon had broken his neck in a fall in 2007. He faced foreclosure on his home last year because he said the injury left him unable to work, but a deal was announced last December that would allow him to stay in the house.
McMahon was Carson's announcer and second banana on "Tonight" from 1962 until Carson's retirement in 1992, introducing him with a resounding "H-e-e-e-e-e-ere's Johnny!" and bolstering the host with hearty laughs.
Carson died in 2005.
McMahon also appeared in commercials for American Family Publishers' sweepstakes, and had supporting roles in movies including "Fun with Dick and Jane" (1977) and "Just Write" (1997) and in TV shows including the 1997 WB sitcom "The Tom Show" with Tom Arnold.
By AP Television Writer Lynn Elber