Fanja, which last won its ninth title in 2014, got off to a dream start in front of its large number of home supporters as its African import, Mboto, leapt high to nod in a powerful header for the opening goal off a corner kick in the eighth minute.
The hosts surged ahead again in the 26th minute following a defence-splitting pass that had Al Nawras al Farsi’s shot beat the Mirbat goalkeeper.
And a minute later, a Mirbat defender's poor back pass off a header was seized by Fanja's onrushing Naseeb al Ghailani to give his club a 3-0 lead.
However, Mirbat captain Hussain al Hadhri reduced the margin in the first-half injury time. And the Salalah-based outfit got a crucial second away goal in the injury time of the second half through Musallam Akaak. The return-leg will be played in Salalah on March 17.