Wallace composed himself to send a measured right-foot shot high into the net and beat home goalkeeper Luke McCormick to cancel out Conor Hourihane's 21st-minute opener.
Argyle's skipper was gifted the opening goal as Portsmouth full-back Yassin Moutaouakil failed to deal with Rommy Boco's cross from the right at the far post.
Hourihane pounced for his third goal of the season as Argyle were rewarded for their early dominance.
The goal acted as a catalyst for Guy Whittingham's visiting side, in front of the 1,405 travelling supporters in a 8,743 crowd, with Wallace and John Marquis on target from range.
McCormick had to be on his toes to palm over a rising Marquis 20-yard shot but could do nothing about Wallace's leveller.
Both sides had opportunities to forge ahead in the second half with Pompey keeper Trevor Carson saving well from Argyle substitute Tyler Harvey in the 86th minute.