Penryn took the lead through Josh Hoon, only for Port to equalise after half time thanks to a Freddie Williams strike.
But late goals from Kevin Lawrence, Ed Wilton and Tyler Cooke sealed victory for Steve Jewell's side.
Pictures: Phil Ruberry
Penryn's Kevin Lawrence breaks clear of both Jed McCardie and Thomas Hawkins
Penryn defender Russell May makes a clearance under pressure
Porthleven's Freddie Williams shields the ball from Penryn midfielder David Broglino
Port's Dan Tarrant challenges Greg Hobbs for the ball
Penryn celebrate Tyler Cooke's injury time goal to make it 4-1