Saints blew away already-relegated Raith in six early first half minutes. Stewart Kean, the last man to score for St Mirren in the league for almost five hours, began the siege on the visiting goal, firing over as a he burst through on Hugh Murray.
Murray's corner was nodded on by Kirk Broadfoot for Kean, whose header was cleared off the line by Paul McMullan.
Kean started the move for the opener in the 14th minute. He released Alan Reid down the right flank, and the full-back's firm measured cross to the far post was flashed into the net by Laurie Ellis with a diving header.
Kean fired wide as Raith were remorselessly put on the rack, and Allan Russell fired in a spetactular second two minutes later.
The striker picked up a throw-in and advanced a couple of strides before unleashing a lethal strike from 35 yards past helpless David Berthelot in the Raith goal.
Russell made the third. He advanced on the right and sent over a precise cross for Kean to volley in from six yards. Scott Crabbe fired over in a lone Rovers attack, but the veteran toiled in vain all afternoon as his young teammates faild to produce the form which had surprised promtion certainties Falkirk.
Russell and Ellis had early second half chances but Berthelot responed alertly, and the rest of the match allowed Raith to recover some dignity - although the home support were left frustrated.