He was one of several amateur club players brought into the provincial set-up at the start of 2014 after the professional side was declared bankrupt. He was included in their squad for the 2014 Vodacom Cup competition and made his debut in their opening match against a. Border lost the match 24–46, but Mapimpi marked his first class debut with a try shortly after half-time. He also started in their 6–60 loss to Eastern Cape rivals and their next match against Kenyan invitational side, scoring the Border Bulldogs' second try in that match to help them to an 18–17 win, their only victory of the competition. He also started the remaining four matches in the competition, scoring his third try of the campaign in their match against the, as the Border Bulldogs finished bottom of the log. He was retained for their 2014 Currie Cup qualification campaign and he made his debut in the Currie Cup competition by starting their opening-day 5–52 defeat to. He missed their next match against the, before starting their other four matches in the competition. However, the Border Bulldogs lost all six of their matches to finish bottom of the log and qualified to the 2014 Currie Cup First Division. Mapimpi once again started all five of their matches, including their first match against the, which turned out to be the Border Bulldogs' only victory of the season as they ran out 19–14 winners. He scored tries in consecutive matches against the and a side that eventually finished top of the log but could not prevent the Bulldogs from finished bottom of the log with a single win all season. Mapimpi returned in the 2015 Vodacom Cup, but ended up on the losing side in their first six matches in the seven-match competition. Mapimpi was the star performer in their final match of the season against the in Alice; he scored a hat-trick of tries and, with regular kicker Masixole Banda unavailable for this match, also took over the kicking duties and slotted three conversions and a penalty. He finished the match with a personal haul of 24 points in the Border Bulldogs' 29–5 victory to help them overtake the Boland Cavaliers on the Southern Section log. He also scored a try three minutes from the end of the Border Bulldogs' first match of the 2015 Currie Cup qualification tournament, with his side causing an upset by beating – the team that won the qualification tournament in 2014 to earn a spot in the 2014 Currie Cup Premier Division – 20–13 in a match played in East London.
Free State Cheetahs
In April 2017, the announced that they contracted Mapimpi until the end of the 2018 season.
Springbok
He made his test debut for the Springboks on 2 June 2018 against Wales in Washington, D.C. at the age of 27 years. He was in the team that won the 2019 Rugby World Cup under the leadership of Siya Kolisi, where he scored 6 tries including two in the 2019 Rugby World Cup quarter final match against in a 26–3 victory. and one in the final against England. Mapimpi is the first player in the history of Springboks to score a try in a Rugby World Cup final. The previous Springboks World Cup wins were without tries.
