The Hoops midfielder began the demolition when his shot was deflected into his own net by Perth midfielder Liam Craig in the 28th minute and four minutes later Commons grabbed number two before setting up Gary Hooper for number three before the break.
The Scotland international scored from the spot 10 minutes into the second-half before substitute Charlie Mulgrew made it five.
CELTIC: Forster; Izaguirre, Ambrose (McCourt 71), Wilson, Lustig; Brown (Matthews 64), Commons, Wanyama; Ledley; Watt, Hooper (Mulgrew 46).
ST JOHNSTONE: Mannus; MacKay, Anderson, McCracken, Miller; Millar, Davidson (Robertson 64), Craig, Moon; Tade (Hasselbaink 74),Vine (MacLean 46).
Quarter-Final: Aberdeen 2 St Mirren 2 (after extra-time, 90m 2-2). St Mirren won 4-2 on pens.