Mundle scored the winner against Burnley last weekend and the 21-year-old was back on target at Fratton Park.
Sunderland were already two up through Zak Swanson's own goal and Alan Browne's cool finish when Mundle netted 11 minutes after the restart.
Luke O'Nien's late own goal couldn't stop Regis Le Bris' team cruising to their fourth win in four league games this term.
Jayson Molumby gave second placed West Bromwich Albion a 1-0 win over Swansea at the Hawthorns to leave them two points behind Sunderland.
Blackburn's Andreas Weimann denied Burnley in a 1-1 draw against their arch rivals at Turf Moor.
Weimann's 25-yard drive cancelled out Lyle Foster's opener as Rovers held on despite playing for more than half an hour with 10 men following Makhtar Gueye's dismissal for a second bookable offence.
Second-half goals from Mateo Joseph and Joel Piroe fired Leeds to a 2-0 home win over Hull to end the visitors' unbeaten start to the season.
Middlesbrough piled on the misery for bottom of the table Cardiff as Matt Clarke's strike and an Aaron Ramsey own goal secured a 2-0 win in south Wales.
Promoted Oxford beat 10-man Preston 3-1 as Emil Riis Jakobsen's third minute opener for visitors was erased by strikes from Mark Harris, Tyler Goodrham and Greg Leigh.
Preston's Liam Lindsay was dismissed in the 70th minute for a second bookable offence.
Derby, promoted alongside Oxford last season, were equally impressive as they beat Bristol City 3-0 thanks to goals from Kenzo Goudmijn, Kayden Jackson and David Ozoh.
Borja Sainz's deflected effort handed Norwich a 1-0 win at Coventry as the Canaries ended their wait for a first league win this season.
Middlesbrough loanee Josh Coburn scored on his debut in Millwall's 3-0 victory over Sheffield Wednesday at The Den.
Duncan Watmore and Jake Cooper were also on target for Millwall, whose first league win this season condemned Wednesday to a third successive loss in the second tier.
Wayne Rooney's wait for a first league win as Plymouth manager goes on after Stoke's Million Manhoef fired Stoke to a 1-0 win at Home Park.
