Bale, back from a loan spell at Tottenham, was in former Everton boss Ancelotti’s first starting XI.
Eden Hazard set up Benzema’s opener with a nice flick and Nacho doubled the lead with a volley.
Benzema scored again and Joselu slotted a penalty for Alaves before Vinicius Jr scored a late fourth for Real.
David Alaba made his Real debut at left-back after his free-transfer move from Bayern Munich – and set up that late Vinicius goal.
Real laboured in the first half but cruised after the break, with Hazard looking in good shape following the fitness problems that have plagued his time at Madrid so far.
A backheel pass by the Belgium playmaker for Benzema’s opener in particular was outstanding.
Bale’s Real Madrid career looks revived after the appointment of Ancelotti, having struggled to get in Zinedine Zidane’s team before going to Spurs for a year. His previous Real game was against Mallorca in June 2020.