McCourt's ownership takes effect immediately, arriving to mark the start of a new era after 20 years.
US tycoon Frank McCourt on Monday officially completed his purchase of Marseille and vowed to invest 200 million euros ($220 mn) to restore the fallen French Ligue 1 giants to their former glory.
The 63-year-old former owner of the Los Angeles Dodgers signed the necessary paperwork in Paris having entered into exclusive discussions to buy Marseille from owner Margarita Louis-Dreyfus on August 29.
Louis-Dreyfus took control after the death of her husband Robert in 2009, but despite a multi-million euro investment the club have struggled finishing 13th last season.
McCourt's ownership takes effect immediately with his new club president Jacques-Henri Eyraud, arriving at the team's Velodrome stadium to mark the start of a new era after 20 years under the control of the Louis-Dreyfus family.
"Today a new chapter opens in the great history of Olympique de Marseille," McCourt said in a statement after penning a deal worth 45 million euros ($49.5 mn).
"The challenges which OM has had to face on and off the pitch had a negative impact on fans of the club in recent years," he continued. "We're going to put our club back on the road to glory."
McCourt told French sports daily L'Equipe later Monday he planned to invest "200 million euros over the next four years" in the club.
The challenges are enormous - Marseille, struggling to attract top players and investment, have not played elite Champions League for the past three seasons.
Fans have been staying away in droves from the Velodrome with less than 25,000 attending last Sunday's 1-0 victory over Metz, the final game of the Louis-Dreyfus era.
Marseille's first game under McCourt's tenure is a daunting test against Qatari-backed French champions Paris Saint-Germain on Sunday.
Eyraud signed his part of the deal in Paris before travelling south to Marseille, arriving at the club's training ground by motorbike taxi where he met the players and staff.
The duo McCourt-Eyraud will be looking to overhaul the former European champions who had the club's worst league finish in 15 years last season.
European Cup winners in 1993, Marseille won the last of their nine Ligue 1 titles in 2010. Coached by Franck Passi they are struggling in 12th in the Ligue 1 this term.
The Boston-born McCourt becomes the latest American to take control of a European football club after Manchester United, Arsenal, Liverpool and Roma all have US owners.
First on their agenda will be to restructure the club with a new sporting director and a finance and marketing chief.
McCourt could try to purchase a big-name player during the winter transfer window and a coach by next season at the latest.
But if McCourt's turbulent eight-year reign over the Los Angeles Dodgers is anything to go by, fans in Marseille could be in for a bumpy ride.
McCourt's Dodgers' reign was marked by friction with Major League Baseball, who eventually seized control with commissioner Bud Selig citing "deep concerns regarding the finances and operations" of the team.
With McCourt struggling to meet the Dodgers' payroll demands, the team filed for bankruptcy protection. The beginning of the end came in November 2011, when McCourt agreed to sell the Dodgers in a process supervised by a federal bankruptcy court judge.
On the pitch during McCourt's 2004-2011 tenure the Dodgers reached the playoffs four times but never progressed past the second round.
He remains the owner of the successful Los Angeles Marathon and in 2014 took a 50 percent stake in the Global Champions Tour show jumping series.