Batting at number nine, Mendis contributed 35 in Sri Lanka's 378 all out at the Galle International Stadium.
The off-spinner then returned to wreck Pakistan's middle order and the tourists finished day two on 191-7, still 187 behind.
Agha Salman (28) led Pakistan's fightback with a defiant 62 before being dismissed in the final delivery of the day.
Yasir Shah (36) survived 61 balls, and an lbw decision against him that was reversed, for his 13 not out.
Earlier, Pakistan fast bowler Naseem Shah (19) and spinner Yasir split the last four Sri Lankan wickets after the hosts had resumed on 315-6.
Niroshan Dickwella (29) struck 51 and Mendis proved he was no dud with the bat to frustrate Pakistan.
Pakistan stuttered early in their reply losing Abdullah Shafique (22), whose unbeaten 160 had secured their two-wicket victory in the series opener, to the second ball of the innings.
Shafique dragged the ball onto his stumps after an ugly poke at an Asitha Fernando (24) delivery to be dismissed for a duck.
Pakistan captain Babar Azam (27), who made 16, departed in a similar fashion, trying to drive a Prabath Jayasuriya delivery without getting to the pitch of the ball and creating an inside edge that crashed onto his stumps.
Opener Imam-ul-Haq (26) made 32 before Dhananjaya de Silva (30) breached his defence.
Mohammad Rizwan (30) and Fawad Alam (36), both made 24, also got the starts but could not convert them with Mendis trapping both lbw.
Rizwan also wasted a review trying to overturn the lbw decision against him.
Mendis removed Mahammad Nawaz (28) to reduce Pakistan to 145-6 but Salman refused to throw in the towel.
The right-hander hit four fours and a six but could not survive the day and was snared in the slip off Jayasuriya's bowling.
