Using their bench to good effect, London made a game of it in the second half but Down always held the aces in this Newry qualifier.
Down motored to a 0-12 to 0-03 half-time lead with defender Conor Garvey popping up with two points.
Paul Geraghty showed London the way, bounding forward to land a point after just 28 seconds, but free-taker Kevin Downes proved to be the Exiles' only other scorer in the first half.
In an interesting aside, Down wing-forward Danny Hughes was lining out against his own brother, Johnny, the attack-minded London defender.
The stand-out player in the opening half was Down midfielder Dan Gordon who dominated his area of the pitch and got forward to grab two well-taken points.
Adding to Down's healthy tally, Paul McComiskey (0-02), Hughes (0-02) and John Boyle (0-03) were also in the scoring mood.
Things would have been worse for the visitors, turning around at the break, but for some fine goalkeeping from Brian McBrearty who saved certain goals on three occasions.
London freshened things up for the second half, introducing three substitutes and they had an immediate effect.
Padraig Duffy got on the scoresheet for them and then Kevin McMenamin had a shot saved by Down netminder Brendan McVeigh, before substitute Chris Donnellan pounced on the rebound to score London's goal.
Donnellan’s strike cut the gap to four points – 0-14 to 1-07 – and that was as good as it got for the valiant Exiles side.
Nine minutes from the end Benny Coulter went in behind his midfielder, won possession with Ronan Sexton and Hughes involved in the movement, before Coulter got the return pass.
The efficient Hughes squared the ball across the London goal for Coulter to regain possession and palm the ball to the net.
A Ronan Sexton point finished the scoring, deservedly putting Down into tomorrow’s second round draw and extending London's 32-year wait for a Championship win.