Two tries from Jonah Holmes put Yorkshire ahead, before Alex Davies crossed against his former club to give the visitors a 24-3 lead at the break.
Will Harries and Curtis Wilson scores brought Ealing, who finished third behind second-placed Yorkshire in the table, back into the match at 27-16.
But Ryan Burrows' try gave Yorkshire an 18-point first-leg advantage.
Should they overturn the deficit and go on to win the final, Ealing, as well as fellow play-off side Doncaster, have said they would not accept promotion to the Premiership.
Doncaster face London Irish, who finished top of the Championship, in the first leg of the other play-off semi-final on Sunday.
Yorkshire Carnegie took control of their Championship play-off semi-final with a first-leg win at Ealing.