Police said Lane, 47, was found around 5:30 p.m. in a hotel room in the San Fernando Valley, which is north of Los Angeles.
An autopsy is scheduled for Friday, Los Angeles County coroner's spokesman Craig Harvey said.
Lane's death is being treated as a possible accident as opposed to one resulting from natural causes, but Harvey said weeks of toxicology and tissue testing may be needed before any conclusions can be drawn.
With his long blond hair and tight leather outfits, Lane embodied the excess of 1980s "hair metal" rock bands. He joined Warrant in 1984 and wrote such hits as "Heaven," "Down Boys" and "Cherry Pie."
He had an on-and-off relationship with the band, leaving it in 1992 before returning and quitting again several times.
Warrant posted a message on its website Friday saying it was "deeply saddened" to learn of Lane's death.
"Our thoughts and prayers go out to his family," the statement said. "Jani was a very important part of our lives for a long time. We will always be incredibly proud of the music we created together. He was a true talent and will be missed by all of us."
In recent years, Lane appeared in VH1's "Celebrity Fit Club" and made news for a drunken driving arrest.
He is survived by two daughters from two previous marriages.