Peterborough United Piergianni was playing in the Peterborough and District Youth League when he was scouted by Peterborough United. He scored his first goals in senior football in a 3–3 draw with
Hayes & Yeading on 5 November 2011, scoring twice to give Stockport a 3–1 lead in the match. He made 44 appearances in all competitions during the
2011–12 season, his first full season of regular first-team football, scoring four goals as Stockport finished the season in 16th position in the Conference Premier. Despite this, he played in three matches in the opening month of the
2012–13 season. He left Stockport at the end of August 2012, with
Director of Football Jim Gannon stating the club were no longer in a position financially to offer the defender a new contract.
Corby Town After leaving Stockport, Piergianni joined
Conference North club
Corby Town on a non-contract basis on 10 October 2012. He debuted for Corby in the club's 5–2 home defeat to Altrincham on 13 October 2012, scoring a consolation goal in the second half of the match.
Boston United Piergianni joined fellow Conference North club
Boston United on 17 May 2013, becoming the club's third acquisition ahead of the new season. He made his Boston debut in a 4–1 victory against former employers Stockport County at
Edgeley Park on 17 August 2013. He was voted Boston's Players' Player of the Year at the club's end-of-season awards ceremony, in a season that saw the club finish in sixth place in the Conference North league table. He was also named in the Conference North Team of the Year that season. Piergianni signed a new two-year contract in May 2014. He was named the club's Player of the Year in May 2015. They were defeated in the semi-final by
North Ferriby United, losing 3–2 on aggregate. Piergianni signed for
South Melbourne of the
National Premier Leagues, Australia's second division, on 12 January 2017.
Salford City Piergianni returned to England and signed a two-year contract with
National League North club
Salford City on 19 May 2017. He made his debut in the club's first game of the
2017–18 season, a 2–0 home defeat to
Darlington. He was voted the club's Player of the Year in April 2018 and was also named in the National League North Team of the Year that season. Piergianni signed a contract extension on 17 December 2018, with the new agreement running until 2021. He played 53 times during the
2018–19 season as Salford earned back-to-back promotions into the Football League after winning the
2019 National League play-off final, a 3–0 victory against
AFC Fylde at
Wembley Stadium on 11 May 2019. He scored Salford's second goal in the match: a header from a
corner kick in the second half. He was named in the National League Team of the Year for the 2018–19 season.
Oldham Athletic Wanting to play first-team football, Piergianni joined fellow League Two club
Oldham Athletic on 6 January 2020, on a loan agreement until the end of the 2019–20 season. Oldham manager
Dino Maamria highlighted the centre-back's leadership skills and character when the signing was announced. He made 11 appearances during the loan spell before the League Two regular season was curtailed due to the
COVID-19 pandemic in March 2020. He scored five times in 45 appearances during the
2020–21 season as Oldham finished 18th in League Two. In April 2021, he won the PFA Community Champion award for the 2020–21 season for his work with the Oldham Athletic Community Trust (OACT) throughout the year. Piergianni was named club captain during the
2021–22 season. He played 48 times during the season, scoring four goals, as Oldham were relegated to the National League. He won the PFA Community Champion award for the second consecutive season in recognition of his continued work with the OACT. With his contract set to expire, Piergianni was released by Oldham at the end of the season.
Stevenage Piergianni signed for League Two club
Stevenage on a free transfer on 23 May 2022. He made his debut in a 2–1 away victory over
Tranmere Rovers on 30 July 2022 and scored his first goal in a 1–0 win against Rochdale on 16 August 2022. He made 55 appearances in all competitions during his first season, scoring eight goals from central defence, as Stevenage secured promotion to
League One with a
second-place finish. At the 2023 EFL Awards, he was named in the
League Two Team of the Season, and was also voted Stevenage
Player of the Year at the club's end-of-season awards. He was included in the club's retained list for the
2023–24 season, signing a new contract on 5 June 2023. Piergianni made 50 appearances that season, scoring four goals, while leading the league in aerial duels won (380) and recording the second-highest number of interceptions (68).{{cite news ==Style of play==