Parcells was introduced by the team Thursday, after more than a week of speculation and conflicting reports.
The two-time Super Bowl winner was hired three days after Cowboy owner Jerry Jones fired Dave Campo, saying he wanted a proven coach and a "change in philosophy."
"I'm energized, determined and I look forward to being part of the community here in Dallas," said Parcells, wearing Cowboy colors -- a silver and blue striped tie. "I know what station football occupies on all levels in the state of Texas."
Parcells, who got a four-year contract reportedly worth $17.1 million, will try to turn around a team that went 15-33 in the last three seasons. He is the first Cowboy coach with experience as an NFL head coach.
Dallas hasn't had a winning record since 1998.
"I look forward to getting the team back to a place of prominence, a place of high competitiveness," he said. "I became convinced that this was the opportunity at this time that was right for Bill Parcells."
Jones described Parcells as the "most qualified coach in our sport that you could draw up if you were drawing your own Rembrandt."
He led the Giants to their only two Super Bowl victories, took the Patriots to the NFL title game and the Jets to the AFC championship game.
After leaving the Jets, Parcells said he never wanted to coach again. He even wrote a book: "The Final Season: My Last Year as Head Coach in the NFL."
That was seven years after he took the Patriot job, saying that would be his "last coaching job, without question."
Then he signed a contract with the Tampa Bay Buccaneers last year but changed his mind. He'd previously backed out of an agreement with the Buccaneers in 1992.
Parcells will have more authority in Dallas than anyone since Jimmy Johnson, the first coach Jones hired after buying the team in 1989. He will also be the first Dallas coach to make more than $1 million a season.
Jones and Parcells make for an unlikely pair.
Jones is also the team's general manager and has relished total control of the organization. Parcells is an unyielding taskmaster who expects authority over his coaching staff and the final say on the roster.
But both men want to get back to the Super Bowl.
Parcells said he agreed to a "partnership" with Jones and that he accepted the owner's having the final say in many decisions -- although neither man gave details of how they will work together.
"There will be changes here, there's no doubt about that," Parcells said.
He declined to say whether he would invite Emmitt Smith to return, or whether he would bring in a new quarterback.