The Wimbledon champions swatted aside Yuichi Sugita and Yasutaka Uchiyama 6-3, 6-4, 6-4 after Richard Gasquet and Gilles Simon had dominated the opening day's singles in Tokyo on Friday.
"We didn't play our greatest match but did what we had to do to win," Mahut told reporters after helping France secure a spot in the Davis Cup quarter-finals.
"It's rare you get easy matches in the David Cup," he added, struggling to be diplomatic after second lopsided day of tennis.
"We had a few complicated moments but we managed to deal with them. We've been together for over three weeks so we know each other well and have this strength."
Japan have failed to win a single set in three matches in the absence of world number five Kei Nishikori, who opted to sit out the World Group clash.
Gasquet swept past Taro Daniel 6-2, 6-3, 6-2 on Friday before Simon overpowered Yoshihito Nishioka 6-3, 6-3, 6-4 to put Yannick Noah's team firmly in the driving seat.
"It was quick and well played, yes," said Noah, a losing Davis Cup finalist in 1982.
"Three-zero against a Japan team without Nishikori looks normal," added the former French Open champion, whose team will face either Britain or Canada in April's quarter-finals.
"And the doubles -- number one in the world against 300, you expect this on paper. But you still have to get the job done. We will go out and party tonight!"
Nine-time winners France have now won all four of their Davis Cup meetings with Japan, who will look to avoid a whitewash in Sunday's dead-rubber reverse singles.
Japan captain Minoru Ueda doffed his cap to the victorious French.
"You could feel the strength of the French team," he said. "It's obvious they have a strong desire to go all the way and win the Davis Cup. Their doubles pair were superb, they never looked like faltering."
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