The 18-year-old, Jamaican-born defendant responded, "Not guilty," in a clear voice each time he was asked for his plea to two counts of capital murder and to one count of using a firearm in a felony.
He is accused in the slaying of FBI analyst Linda Franklin, who was killed by a single bullet outside a store on Oct. 14, 2002.
His lawyers are arguing he was brainwashed by fellow suspect John Allen Muhammad, 42, who is standing trial in another city nearly 15 miles away.
In contrast to Muhammad's usual stony demeanor, Malvo smiled frequently and occasionally laughed as he conferred with his attorneys. His lawyers plan to pursue an insanity defense.
Wendell Odom, the lawyer for Andrea Yates, the woman who claimed she drowned her five children in a bathtub on orders of the devil, calls the insanity plea a legal long shot, reports CBS News Correspondent Thalia Assuras.
"There is just a distrust towards not holding somebody criminally culpable because they are sick," he said.
The insanity defense is used in only 1 percent of cases and only a quarter of those succeed. So, many analysts believe Malvo's defenders are aiming not for an acquittal, but to avoid the death penalty by showing Malvo was under a spell, reports CBS' Assuras.
Malvo and Muhammad are being tried for different killings; Muhammad is accused of gunning down a man at a gas station.
Both cases were moved away from the nation's capital out of concern that an impartial jury could not be found close to Washington because the shootings caused such widespread fear. Malvo's case was moved to Chesapeake, Muhammad's to nearby Virginia Beach.
In Muhammad's case, prosecutors called their final witness after three weeks of emotional testimony from victims and witnesses about the sniper spree that killed 10 and wounded three in October 2002.
Muhammad's lawyers want the charges thrown out, saying prosecutors offered no evidence that he pulled the trigger in the slaying for which he is on trial. Prosecutors contend Muhammad exerted such control over Malvo that Muhammad should be held responsible even if Malvo pulled the trigger.
Similarly, Malvo's lawyers plan to argue that he was so "indoctrinated" by Muhammad that he either did not know what he was doing or could not control himself.
Twelve jurors plus four alternates will be selected from a pool of 151 potential jurors for Malvo's trial, which is expected to last six weeks. Forty-one people were quickly dismissed for various reasons.
Circuit Judge Jane Marum Roush denied a defense request to dismiss a capital murder charge that accuses Malvo of committing terrorism when he allegedly shot Franklin.
"Prosecutors hold all the cards. They have evidence. They apparently have a confession. This is a defendant who is not going to generate a lot of sympathy because of what he said about his victims," says CBS Legal Analyst Andrew Cohen.
In Virginia Beach, Circuit Judge LeRoy F. Millette Jr. sent the jury home after the prosecution wound up its case in Muhammad's trial. The defense is expected to begin presenting its case on Wednesday.
While Malvo and Muhammad are on trial in one killing each, to win a death sentence prosecutors must show that the two took part in multiple killings or terrorized the public.