The guard received stitches and was released from the hospital Tuesday.
Vance declined to identify the guard or inmate, one of approximately 415 prisoners at the maximum-security facility. State police are investigating the attack to decide whether to charge the inmate.
The lockdown was expected to be lifted later Wednesday, meaning inmates would no longer be confined to their cells and visitations would resume.
Northern is the state's highest security prison, housing 10 death row inmates as well as prisoners who officials say "have demonstrated a serious inability to adjust to confinement" and who threaten the safety of guards and other inmates.
The warden's office at Northern referred calls Wednesday about the attack to the Department of Correction's public information office, where a spokeswoman said further information was not being released.
Larry Dorman, a spokesman for the AFSCME Council 4 union - which includes prison guards among the 17,000 state workers it represents - calls the slashing "the latest demonstration of how dangerous and volatile working conditions can be in the state's prisons."
"Our members are very dedicated to the work they do, but this is emblematic of the risks they face to their lives, to the lives of the people inside the facility and the public at large," he said.
The facility has been on lockdown several times over the past few months. In September, four inmates attacked three guards in separate incidents. A few weeks later, an inmate set his mattress on fire, forcing guards to temporarily relocate about 40 prisoners.
Earlier this year, officials decided to remove 580 metal lockers from cells at the prison after two inmates made knives from locker parts and stabbed another prisoner. The lockers are being replaced with plastic containers.