Sleightholme signed a senior academy contract with Northampton Saints before the 2018–19 season.[8] He made his debut for Northampton in a Premiership Rugby Cup match against Bristol Bears on 27 October 2018.[9] And he made his Premiership debut three weeks later, scoring his first Northampton try 14 seconds after coming on as a second-half replacement against Wasps.[10][11] In only his second European Rugby Challenge Cup game, he scored four tries against Timișoara Saracens on 18 January 2019.[12]
Sleightholme also played at the 2019 World Rugby Under 20 Championship,[14] and he scored two tries as England lost to Ireland in the opening game.[15] He subsequently scored a try in England's final pool game against Australia,[16] with England eventually finishing the tournament in fifth place.[17]