But two tries from Wales centre Jonathan Davies kept the Scarlets in touch with Regan King also crossing.
Yet Connacht seemed set to hold out until seven minutes into added time Lamont crashed over to send Parc y Scarlets wild.
Meanwhile, Cardiff Blues were on the end of a late comeback as Leinster scored two tries in the final ten minutes to grab a hard-fought 34-23 win.
Wales lock Bradley Davies and Richie Rees were on target for the Blues in the second-half after Dai Young's men turned a 20-6 half-time deficit into a three point lead.
But ultimately they were to be left emptyhanded as late tries from Fergus McFadden and Ian Madigan left the Blues without the consolation of a bonus point.