The 31-year-old from Northern Ireland has been a stand-out performer for Saints and has now been rewarded with a one-year contract extension.
Mannus quit Shamrock Rovers to move across the Irish Sea and sign with St Johnstone in July, 2011.
He has made 44 appearances for Lomas' men this season, clocking up nine clean sheets as he helped the side register the third best defensive record in the SPL.
St Johnstone conceded just 44 league goals this term, with only the Clydesdale Bank Premier League champions Celtic and eighth-placed Aberdeen boasting better figures.
Their solidity at the back was crucial to the club's late bid for Europa League qualification..
Saints snatched third place in the SPL and with it qualification for European competition with their 2-0 win over Motherwell in Sunday's final round of league fixtures.
That result, coupled with Inverness' 1-0 defeat to Ross County, means St Johnstone will be back on the continent for the second season running.
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