The Green Lions struggled to lay a glove on the newly-promoted side throughout the first period, and it was only some Jason Martin brilliance on the stroke of half-time that denied Bruno Tavares from close range to keep it level at the break.
GFC managed to keep the hosts at bay until midway through the second half, before a right-footed Luke Heneghan curler into the top corner provided Southall with a well-deserved opener.
They soon doubled their lead, too, as Arell Amu scrambled in a rebound following a Martin save to establish breathing room between the two sides.
Just one minute after Amu’s strike, things went from bad to worse for Guernsey, with the hosts being awarded a penalty in the 66th minute which captain Louis Stead had no trouble converting.
Chances continued to be few and far between for Tony Vance’s men and Liam Mahon was forced to withdraw from proceedings on the 70th minute mark after pulling up injured while trying to stop an advancing Southall player.
Brandon Wallace was his replacement, as the youngster was handed his competitive Guernsey FC debut less than 24 hours after scoring an early goal of the season contender in a 3-1 North win over Saints in their Priaulx League opener.
Danny Hale went close late on with what was Guernsey’s only real clear-cut chance of the afternoon, displaying some impressive acrobatics to meet Robbie Legg’s cross and striking just wide of Michael Edegbe’s goal.
It was only really Southall who ever looked like providing a real threat to the scoreline, though, and the home dugout erupted in rage in the closing moments after being denied a penalty shout.
It ended 3-0 in Middlesex, meaning GFC drop to 18th in the Isthmian South Central, although Vance’s men have played at least one game fewer than every other side in the division bar Sutton Common Rovers.
Victory for Southall means they climb to seventh in the league table, and could rise even higher with a win at league leaders Northwood today.
The Green Lions still remain in the hunt for their first league goal of the season and will have the opportunity to get off the mark once again today when they host Tooting & Mitcham United at Footes Lane. Kick off is at noon.