The Canadian-born Northern Ireland goalkeeper was set to come out of contract at McDiarmid Park this summer but has penned a 12-month extension.
“St Johnstone gave me my chance when no one else had any interest in taking me from Ireland,” he said.
“I felt I owed it to Saints to extend my stay if they wanted me.
“Europe is the icing on the cake because I had already agreed to stay before Sunday.
“Hopefully it will persuade a few of the other lads to stay and St Johnstone will have a bit more money for finishing third.”
Mannus joined the Saints from Shamrock Rovers in 2011 and has worked his way up to the clubs number one keeper, making 38 league appearances this season.