Born in Worthing, Chaplin progressed through Portsmouth's youth categories after joining the club at the age of six. He was offered a two-year scholarship contract on 3 July 2013. 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. He made his League Two debut on 13 December, from the bench in a 2–3 home defeat against Accrington Stanley. Chaplin scored his first professional goal on 11 April 2015, netting his team's only in a 1–3 away defeat against Morecambe. He made his first start for Pompey in their final game of the season, a 1–1 home draw against York City. At the end of the season he was named the League Two Apprentice of the Year.
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, 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. 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.
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. 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, Barnet, Blackpool, Notts County and Cambridge before the end of October – a run of form which would see him established as Portsmouth's first-choice striker. 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.
At the end of August 2018, Chaplin joined Coventry City on loan with a permanent agreement in place for the following January. Chaplin scored his first goal for the club from the penalty spot in a 2–1 away victory against Oxford United. On 19 January Chaplin scored the opener in a 2–1 away defeat to Plymouth Argyle F.C.
Barnsley
On 19 July 2019 Chaplin joined Barnsley for an undisclosed fee on a four-year deal. On 14 December Chaplin scored his first career hat-trick in a 5-3 Barnsley victory over QPR at Oakwell.
