The striker stabbed home his second goal of the campaign at the end of a second half which the hosts dominated.
It was Betis' second straight 1-0 success after Ruben had left it even later to seal an opening-day win at Granada.
Benat fired the first chance of the game over the bar for Betis before Mallorca began to dictate the game.
Alejandro Alfaro shot wide of the left post and then brought a parry out of Betis goalkeeper Casto.
The keeper had to be alert again to keep out Emilio Nsue's effort from the left side of the area, but it was the Mallorca goal which lived a charmed life just before the break.
Jefferson Montero drilled a shot against the post, Ruben headed the follow-up against the bar and then fired another rebound off the bar from six yards as Betis somehow failed to break the deadlock.
The initiative was with the hosts now, though, and Montero's close-range effort was tipped round the post by keeper Dudu Aouate moments later.
Ruben headed straight at Aouate early in the second half, while Benat's shot was blocked by Ivan Ramis.
Isreal international Aouate produced a fantastic save to parry away Jonathan's left-footed shot from the edge of the area before the same player sent a header wide from Benat's free-kick.
But Ruben finally found a route to goal in the 86th minute as he turned home Benat's inswinging corner at the far post to maintain Betis' perfect start to the season.