Ballinamallard were dealt a killer blow in the closing minute of normal time when substitute Mark Patton netted for the visitors to give them a 2-1 win and the three points.
Glenavon had taken the lead after just two minutes when new signing Tiarnan Mulvanna headed home at the far post from a corner.
Ballinamallard though levelled on 29 minutes when Mark Stafford headed home a David Kee free kick.
With the points looking shared, a mistake by Keohane allowed Glenavon to break and from the driven cross, Patton was on hand to stab home.
Ballinamallard United: Rouse, Keohane, Stafford, Kee, Cashel (Lecky), Hutchinson, Crawford (McManus), N. Beacom, Campbell, Martin (Sheridan), Crilly. Subs: Currie, C. Beacom
Glenavon: McGrath, neill, Lindsay, Hamilton (Bates), McGrory, Marshall, McKeown (McCallion), McCabe, Martyn, McCaul (Patton), Mulvenna. Subs: Kilmartin, Rainey
Referee Hugh Carvill