Circuit Judge Christopher Wilkes sentenced Johnson to the mandatory life in prison.
He also gave Johnson a series of five consecutive sentences adding as many as 5 to 30 years to his life term.
Johnson is awaiting trial in Martinsburg, W.Va., on two counts of sexually assaulting a fellow inmate at the Eastern Regional Jail while awaiting trial on the kidnapping offense.
Johnson spoke briefly to the judge before his sentencing. He said he feels no hatred toward Tammy Reel, his co-defendant and the mother of his child, who testified for the prosecution during his trial.
Reel, who is facing the same charges, has not been tried.
He said he is sorry for what his crime has done to his family. He did not mention his victim by name, but said he regretted doing what he did.
"I do feel remorseful for what happened. If we weren't under the influence, none of this would have taken place," Johnson said.
In addition to the kidnapping conviction, Johnson also was found guilty of attempted first-degree murder, bringing stolen property into the state, conspiracy to commit kidnapping, conspiracy to attempt first-degree murder and conspiracy to bring stolen property into the state.
Johnson was convicted in the Aug. 2, 1996, attack on Donald Milligan, 50, of Gaithersburg, after Reel lured Milligan into a room at the Days Inn motel.
Believing Milligan was dead, Johnson stuffed him into the back seat of Milligan's car, and later moved him to the trunk. Then he and Reel spent the night looking for crack cocaine, Reel testified.
The next morning, Johnson threw Milligan, who was playing dead, into the Potomac River, according to testimony.
Johnson said Monday that he turned down a plea agreement for 80 years because he figured he would take his chances with the jury.
"I've seen cases where people shot someone in the head at point blank range and get two to 20 (years)," Johnson said.
Johnson said he was thankful he was convicted in a state that does not have the death penalty.
"I could have received a lethal injection. I'm still living. I thank God for that," he told the judge.
Wilkes denied Johnson's request for a new trial, agreeing with the prosecution that the evidence was strong enough for the jury to have properly reached a guilty verdict.
"I wish every time I had a case to try I had that much evidence," said Jefferson County Assistant Prosecuting Attorney Laurence Crofford.
During the trial, the prosecution introduced evidence gathered at the motel room and the stolen car as well as videotape from Martin's supermarket where Johnson went to get breakfast after dumping Milligan's body in the river. The evidence was all corroborated by Milligan's and Reel's testimony.
Crofford said the fact that Johnson thought Milligan was dead did not excuse kidnapping.
Crofford said Johnson has a history of violent crimes, from battery to obstructing police officers.