The 2018–19 NIFL Championship was contested by 12 teams. Institute were champions in the previous season, and were promoted to the 2018–19 NIFL Premiership. They were replaced by the bottom team in last season's Premiership, Ballinamallard United. Runners-up Newry City won 6–3 on aggregate in the promotion play-off against Carrick Rangers, also achieving promotion to the Premiership for this season and relegating Carrick Rangers to the Championship.
The bottom team from the previous season, Lurgan Celtic, were relegated to the third-tier NIFL Premier Intermediate League. They were replaced by Dundela, winners of the Premier Intermediate League. The eleventh-placed team from the previous season, Dergview, remained in the Championship this season, after defeating the NIFL Premier Intermediate League runners-up Queen's University 4–1 on aggregate in the relegation play-off.
The eleventh-placed team from the Championship, PSNI, faced Annagh United, the runners-up from the 2018–19 Premier Intermediate League over two legs for one spot in the 2019–20 NIFL Championship. PSNI won 5–3 on aggregate to retain their place in the Championship for next season. Annagh United remained in the Premier Intermediate League.