Airdrieonians had been looking to keep themselves ahead of the chasing pack and extend their unbeaten start to the new league season.
The Diamonds were on the back foot after defender Cammy Ballantyne was shown a straight red card in the 16th minute for deliberate handball on the line to block a goalbound effort.
Daniel Handling made no mistake from the penalty spot, with James Craigen adding a second on the stroke of half-time.
John Robertson made it 3-0 after 54 minutes and soon scored another on the hour. Substitute Ryan Shanley chalked up a fifth with 20 minutes left, before Craigen completed the rout late on.
Dunfermline moved two points clear of Edinburgh after a 2-0 win at Queen of the South with first-half goals from Nikolay Todorov and Rhys Breen.
Montrose are third after they won 2-0 despite having forward Rory McAllister sent off at bottom club Peterhead, who suffered a fourth straight league defeat. Matthew Wright and Lewis Milne scored the goals.
Kelty Hearts picked up their first league win after beating Falkirk 2-0 as Joe Cardle and Scott McGill struck before half-time at New Central Park.
Alloa won 4-2 at Clyde to secure back-to-back league victories thanks to goals from Bradley Rodden, Conor Sammon, Kieran Offord and an own goal from Brian McLean.
Meanwhile, Dumbarton maintained their perfect start to the cinch League Two campaign with a 1-0 win at East Fife.
Declan Byrne struck late in the first half to give the Sons a sixth straight league victory.
Newcomers Bonnyrigg Rose remain second, but are now eight points behind after losing 2-0 at Elgin. Bryan Cameron and Kane Hester scored the goals.
Stenhousemuir sit fourth after beating Stranraer 3-1 at Ochilview Park. William Sewell gave the Warriors and early lead, before Nicky Jamieson and Sean Crighton struck twice the space of five minutes before the break.
Tommy Goss grabbed a second-half double as Annan came from behind to win 2-1 against Forfar, while Stirling and Albion drew 1-1 at Forthbank Stadium, where Kyle Banner’s opener early in the second half for the Binos was soon cancelled out by Charlie Reilly.