The 2018–19 season was the club's first season of play back in the top tier of Scottish football since 2015, having been promoted from the Scottish Championship at the end of the previous season. The club had been relegated from the Premiership at the end of the 2014–15 season. St Mirren will also compete in the League Cup and the Scottish Cup.
Month by month review
April
30 April – the club announced that Stelios Demetriou, John Sutton, Gary Irvine, Massimo Donati, Josh Todd, Darryl Duffy, Darren Whyte and Nathan Flanagan would leave the club when their contracts expire, while Lewis Morgan, Mark Hill and Liam Smith would all return to their parent clubs following the expiry of their loans.
May
2 May – young midfielder Cameron MacPherson signed a six-month contract extension until January 2019, after becoming a regular in the development squad. 4 May – goalkeeper Craig Samson signed a contract extension, keeping him at the club until the summer of 2020. 9 May – experienced defender Gary MacKenzie signed a one-year contract extension, tying him to the club until 2019. 11 May – defender Harry Davis left the club, after rejecting a new contract offer. The popular Englishman cited family reasons for wanting to move back down South. In total Davis played 31 games for Saints, netting 6 goals. 14 May – manager Jack Ross was awarded the Scottish Championship Manager of the Year award for season 2017–18. 22 May – midfielder Jim Kellerman signed a two-year deal with Saints, after leaving National League side Aldershot Town. Also on this day, youth academy player Ben Cameron signed for the club on a two-year development contract. 25 May – Saints announced that manager Jack Ross and his assistant James Fowler had left the club to become the new management team at Sunderland. The pair signed two-year deals with the League One side.
Also on this day, Saints signed defender Paul McGinn on a two-year deal from relegated Partick Thistle. McGinn is the brother of Saints captain Stephen McGinn, and previously left Saints in 2013 without playing a game for the club.
Also on this day, Saints were drawn against Kilmarnock, Dumbarton, Queen's Park and Spartans in the group stage of the Betfred Cup.
June
8 June – Alan Stubbs was appointed as the club's new manager, signing a three-year deal. 21 June – Saints made a double signing, with defender Josh Heaton and forward Cody Cooke joining the club. Highly rated Heaton signed on a three-year-deal from Darlington for a reported fee of £75,000. Cooke signed a two-year contract, and joined from Truro City. 22 June – midfielder Jeff King signed on a two-year deal from Bolton Wanderers. 23 June – young midfielder Kyle Magennis signed a one-year extension to his current deal, keeping him contracted to the club until 2021. Also on this day Brian Rice was appointed as the club's new Assistant Manager.
July
3 July – last season's top scorer Gavin Reilly left the club on a free transfer, signing for Bristol Rovers. 9 July – Saints signed young left back, Hayden Coulson, on a season-long loan from Middlesbrough. 11 July – backup goalkeeper Ross Stewart left the club, signing for Livingston on a two-year deal. Saints then moved quickly to sign Aberdeen goalkeeper Danny Rogers on a season-long loan. 13 July – forward Myles Hippolyte left the club on a free transfer, signing for Dunfermline Athletic on a two-year deal. Hippolyte spent six months with Saints, scoring one goal in eight appearances. Also on this day, defender Cole Kpekawa signed a two-year deal after leaving Colchester United. 27 July – midfielder Matty Willock joined on a season-long loan from Manchester United.
August
9 August – Danish striker Nicolai Brock-Madsen signed for Saints on a six-month loan from Birmingham City, with an option of a further six months. 10 August – forward Ross C. Stewart left Saints for an undisclosed fee, signing for Scottish Championship side Ross County. He scored three goals in twenty-one appearances for the club. 15 August – 20-year old defensive midfielder, Alfie Jones, signed on a season-long loan from Premier League side Southampton. 17 August – midfielder Jim Kellerman joined National League side AFC Fylde on a season-long loan, just months after signing a two-year contract with the club. He has made four appearances since joining. Also on this day, youngster Ethan Erhahon signed a three-year extension with the club. 20 August – defender Lee Hodson joined the club on a season-long loan from Premiership rivals Rangers. 31 August – midfielder Ryan Edwards joined on a one-year loan from Heart of Midlothian.