NFL Commissioner Roger Goodell imposed the suspension because Stallworth violated the league's policies on substance abuse and personal conduct.
Stallworth, who may not participate in any team activity this season, will be reinstated after the Super Bowl in February. His case sparked public outrage because, after pleading guilty to drunk driving and manslaughter in Florida, he was sentenced to just 30 days in jail -- and was released after serving 24.
In a letter to Stallworth, Goodell wrote: "In my view, the essential facts are that you had alcohol in your system well above the legal limit, made a conscious decision to drive, and struck and killed a man. As you recognize, this conduct and the loss of life has caused serious damage to the NFL and NFL players generally. Legal arguments that focus on criminal liability under Florida law do not diminish that damage or your responsibility for your conduct."
Last week, Stallworth said he felt he had done "irreparable harm" to the family of the man he killed and was ready to accept whatever ruling Goodell had in store.
Goodell, who in June suspended Stallworth indefinitely, wrote that he is "clearly guilty of conduct detrimental to the integrity of and public confidence in the NFL."
The commissioner added: "Despite a repeated emphasis on the importance of avoiding driving under the influence of alcohol, you chose to drive under circumstances where you were legally impaired. And you did so even though safe and confidential alternatives, such as the 'Safe Ride' program, were available to you. Your conduct endangered yourself and others, leading to the death of an innocent man. The NFL and NFL players must live with the stain that you have placed on their reputations."
The suspension is twice the eight-game penalty the St. Louis Rams' Leonard Little received from then-Commissioner Paul Tagliabue in 1999, after Little struck and killed a woman while driving drunk.