2014–15 season & senior breakthrough

Chaplin signed a two-year professional deal with Pompey on 20 October 2014, after scoring 11 goals in 11 appearances for the under-18's.^[5] He made his League Two debut on 13 December, from the bench in a 3–2 home defeat against Accrington Stanley.^[6] Chaplin scored his first professional goal on 11 April 2015, netting his team's only goal in a 3–1 away defeat against Morecambe.^[7] He made his first start for Pompey in their final game of the season, a 1–1 home draw against York City.^[8] At the end of the season he was named the League Two Apprentice of the Year.^[9]

2015–16 season

Chaplin started the 2015–16 season on the bench, playing second fiddle to first-choice striker Matt Tubbs, but showcased his talents with goals against Championship clubs Derby and Reading in the first and second rounds of the Football League Cup,^[10][11] also scoring in the 3rd round of the FA Cup in a 2–2 draw with Ipswich Town. He scored his first two league goals of the season in a 3–1 win over Barnet on 12 September.^[12] Although Chaplin struggled to maintain a first-team place in the league, he would still manage to score 11 goals in 38 appearances in all competitions over the course of the season.^[13]

2016–17 season

Competition for the sole striking berth in manager Paul Cook's preferred 4–2–3–1 formation was made all the more intense at the start of the 2016–17 season, following the signings of Michael Smith, who made his loan move permanent, as well as Curtis Main from Doncaster Rovers and experienced forward Noel Hunt.^{[citation needed]}

Chaplin subsequently found himself on the bench in the early weeks of the season, before the inconsistent form of Smith and Main, coupled with Hunt's injury problems, saw Chaplin promoted to the starting lineup. He grasped the opportunity, scoring league goals against Wycombe Wanderers, Barnet, Blackpool, Notts County and Cambridge United before the end of October^[14] – a run of form which would see him established as Portsmouth's first-choice striker.^{[citation needed]}

As the season continued, Cook would rotate his centre-forwards, and Chaplin lost his place to the more physically imposing Smith. Though Smith would ultimately fall out of favour and move to Northampton Town in the January transfer window, the signing of Eoin Doyle – who had previously played under Cook at Chesterfield – ensured that Chaplin would remain on the bench. Chaplin continued to make cameo appearances as a substitute, however, and took his tally to eight for the season with a last-gasp, 25 yard strike against Barnet to snatch a point in a 1–1 draw on 18 February.^[15] Chaplin finished the season with 8 goals in 42 appearances, helping Portsmouth to lift the EFL League Two title.^[16]

2017–18 season

Following Portsmouth's promotion to EFL League One, Chaplin continued to feature regularly. He scored his first goal of the season on 26 August, netting an equalizer in a 1–1 draw with Wigan Athletic. On 10 February, he scored a late winner in a 2–1 away win against Milton Keynes Dons. Chaplin helped Portsmouth establish themselves in League One after an 8th-placed finish, making 32 appearances in all competitions, scoring 5 goals.^[17]