Sampaoli, a much-travelled Argentinean, replaces Andre Villas-Boas, who was suspended from the club on 2 February and has signed a contract until 2023.
McCourt said he wanted to start a "new chapter" and promised "profound changes" at the club, firing president Jacques-Henri Eyraud, who has become a target of fan discontent, and replacing him with sporting director Pablo Longoria.
"This is a new chapter for Olympique de Marseille, and I personally commit myself to making it a real success," McCourt said.
Sampaoli has prior coaching experience at both international and club level. Renowned for his focus in attack, he has led the national teams of Argentina and Chile, as well as Sevilla in LaLiga, and also Santos and Clube Atlético Mineiro in Brazil.
Sampaoli has led Chile to its first major trophy in 2015 when it won the Copa America, beating reigning champion Uruguay in the quarter-finals and Argentina in the final.
"I have been told all my life that OM is a special place for passion, that the Vélodrome lights up when the team plays in the stadium," Sampaoli said after being announced as Marseille's new coach.
"Marseille is a club for the people. We're not here to hide: we're going to play hard. When I received this proposal, I dreamed of being able to party in the city. In the world, there are places for calm and there are intense places. It is the latter that I want and I accepted without hesitation. This club has a soul and that is why we are here. We are ready.
Marseille, which was second in Ligue 1 last season when the the competition was cut short because of the coronavirus pandemic, is currently seventh on the table for 2020-2021, 20 points behind current leader Lille.