[5] He made his first competitive appearance for Burton in a 1–0 defeat to Notts County at Meadow Lane in a Football League Trophy tie on 3 September 2013.

[6] He made his league debut on 22 March 2014, coming on for Alex MacDonald 68 minutes into a 1–0 defeat to Scunthorpe United at Glanford Park.

[7] He made his first league start on 3 May, in a 1–0 defeat to Southend United at Roots Hall; this was his fourth and final appearance of the 2013–14 campaign.

[15] However, Burton recalled him to allow him a chance to play higher-level football, and caretaker manager Andy Watson said that "we were delighted to have had Marcus here and his loan deal wasn't far from ending".

[22] Manager Neil Aspin played Harness just behind striker Tom Pope, in an attacking midfield role that demanded a lot of hard work.

[34] He finished the 2019–20 season strongly, scoring none goals and providing seven assists in 37 games, often making a big impact from the bench; he earned himself praise from Jackett, who said "he has got a very, very good future ahead of him".

[38] On 13 March 2021, he played in the 2020 EFL Trophy final, which had been postponed from the previous year due to the COVID-19 pandemic; he entered the game as a half-time substitute for Jordy Hiwula, which ended as a 0–0 draw, with Salford City winning the penalty shoot-out.

[40] He achieved new manager Danny Cowley's target of ten goals for the season, however, Portsmouth missed out on the play-offs after finishing in eighth-place.

[41] In January 2022, Cowley said the club's priority would be to extend Harness' contract as he was the stand-out performer in scoring ten goals from midfield in the first half of the season.

[42] Harness was though sent off for the first time in his career for fouling Ross Sykes in "a clear act of petulance" during a 4–0 win over Accrington Stanley at Fratton Park on 5 March.

[52] He scored four goals and provided two assists in 39 games as the club secured promotion into the Premier League with a second-place finish at the end of the 2023–24 Championship season.

[59] Burton Mail reporter Joshua Murray described Harness as a pacey and skilful winger, who is right-footed but able to play on either flank or even at wing-back.