The new St. Lucian Prime Minister took the oath of office before Dame Perette Louisy at Government House on the outskirts of the capital.
Among those observing the ceremony were Mr. Chastanet's wife, their two children, his 82 year old father, Michael, and members of his victorious United Workers Party.
He thanked St. Lucians for listening during the campaign trail and exercising their Constitutional rights on Monday.
He promised that the UWP will bring about the change they want.
"We humbly accept the overwhelming mandate you have given to us."
The UWP won 11 of the 17 electoral seats in the St. Lucia General Election. The St Lucia Labour Party (SLP), led by former Prime Minister Dr. Kenny Anthony, got the six remaining seats.