Byfield, sold by Walsall last season for £50,000, struck twice as did his strike partner, Alan Lee.
It was Lee's 77th minute penalty that finally killed off Walsall who had twice fought their way back on to level terms.
Australian international David Zdrilic grabbed his first goals for the club with a double and Jorge Leitao was also on target but it was all in vain as Walsall slipped to a fourth successive League defeat.
Byfield soon made Walsall regret selling him when he fired Rotherham into a 14th minute lead, capitalising on a flick from Lee after a long clearance from the goalkeeper Mike Pollitt.
But Rotherham did not really have time to celebrate Byfield's first strike as Walsall were on level terms 60 seconds later with their first goal in six hours and eight minutes of first-team action.
Left-back Zigor Aranalde fired over a cross that Zdrilic drove past Pollitt at his near post.
Walsall failed to build on quickly forcing their way back to level terms and Byfield left them red-faced again after 22 minutes.
This time he reacted faster than the Walsall defence when Darren Garner whipped in a free-kick and his volley from ten yards whistled past the Walsall goalkeeper, James Walker.
When Lee rose above the Walsall defence after 29 minutes to head home a corner from Garner, Rotherham looked set to cruise to a comfortable win.
But Walsall were handed a lifeline on the stroke of half time when Zdrilic collected a loose ball in midfield.
Zdrilic was allowed to move towards the Rotherham area and then gave Pollitt no chance with a fierce 25-yard drive.
It proved to be a vital strike as Walsall were able to draw level 17 minutes into the second half.
Zdrilic was once again involved, rising above the Rotherham defence to nod a free kick from Darren Bazeley into the path of Leitao.
The Portuguese marksman did the rest, turning smartly to fire a low shot home from ten yards.
But Walsall's battling fightback was to prove to be in vain as Lee converted from the spot after Chris Sedgwick had been fouled by Aranalde.