Hardy, who signed a one-year deal with the Cowboys in March, will appeal the ruling. If the league's ruling stands, the first game Hardy would be eligible to play would be Nov. 26 against his former team, the Carolina Panthers, on Thanksgiving.
Commissioner Roger Goodell wrote a letter informing Hardy of the decision, which came after what the league called an "extensive two-month investigation" after Hardy's case was dismissed in North Carolina state court.
Goodell wrote that a suspension of this length would be appropriate under the current or former personal conduct policy.
In July 2014, Hardy was found guilty of assaulting a former girlfriend, Nicole Holder, and threatening to kill her, and he received a 60-day suspended sentence and 18 months' probation on misdemeanor charges. He appealed the judge's ruling in favor of a jury trial, and the case was dismissed in February when Holder chose not to cooperate with authorities after receiving a settlement from Hardy.
This week Hardy asked the court that his record be expunged.
"This suspension is something that we anticipated prior to Greg's signing, and we respect the commissioner's ruling,'' Cowboys owner Jerry Jones said Wednesday in a statement. "Our organization understands the very serious nature of this matter. We will use our resources -- work closely with Greg and with the league -- to ensure a positive outcome.''
Before reaching its decision, the NFL was able to look at court records, documents, exhibits, photographs, police reports and medical records as well as reports and opinions of medical experts retained by Hardy's attorneys and the NFL.
The NFL said it made "numerous efforts" to interview Holder but was unable to do so. The league also said Hardy failed to provide complete and accurate information to the NFL investigators and the league's staff. The league said Hardy's attorneys did not provide the NFL with a copy of the settlement with Holder or acknowledge that an agreement exists.
The investigation was led by Lisa Friel, the league's senior vice president and special counsel for investigations and a vice president at T&M Protection Resources. Before joining T&M, Friel was head of the sex crimes prosecution unit in the New York County district attorney's office.
Hardy, his lawyer and NFL Players Association representatives met with the NFL staff and investigators on March 4, with independent investigators on March 10 and again recently after the league was able to review certain photographs made available by the district attorney's office in North Carolina.
According to the league, its investigation concluded that Hardy used physical force against Holder in "at least four instances."
"First, he used physical force against her which caused her to land in a bathtub," a league news release said. "Second, he used physical force against her which caused her to land on a futon that was covered with at least four semi-automatic rifles. Third, he used physical force against her by placing his hands around Ms. Holder's neck and applying enough pressure to leave visible marks. And fourth, he used physical force to shove Ms. Holder against a wall in his apartment's entry hallway."
Although Goodell didn't talk with Holder and Hardy was not penalized legally, the commissioner's decision was based "on findings that are supported by credible corroborating evidence independent of Ms. Holder's statements and testimony, such as testimony of other witnesses, medical and police reports, expert analyses, and photographs."
"The use of physical force under the circumstances present here, against a woman substantially smaller than you and in the presence of powerful, military-style assault weapons, constitutes a significant act of violence in violation of the Personal Conduct Policy," Goodell wrote in the letter to Hardy.
Hardy signed a one-year deal with the Cowboys that did not include any guaranteed money but could have been worth up to $13.2 million thanks mostly to per-game roster bonuses totaling $9.5 million. If the suspension holds, the most Hardy could make from the roster bonus would be roughly $3.5 million. He is set to earn a base salary of $750,000. With only six games left after the suspension, it would be difficult for him to reach any of the thresholds in the incentive package that totaled $1.804 million.
Hardy has been working out at Valley Ranch for most of the past month. Per league rules, he can participate in the Cowboys' offseason workout program, organized team activities, minicamp, training camp and preseason games. The suspension will begin Sept. 5.
The league has directed Hardy to undergo a clinical evaluation by a professional of his choosing. If counseling is recommended, Hardy would be expected to comply and provide evaluations to the NFL.
"You must have no further adverse involvement with law enforcement and must not commit any additional violations of league policies," Goodell wrote. "In that respect, you should understand that another violation of this nature may result in your banishment from the NFL."