On 16 June 2014, Ainsworth signed for Motherwell permanently on a one-year deal. In the UEFA Europa League second qualifying round second leg match away against Stjarnan, Ainsworth scored his first European goal, but Motherwell lost 3–2, being eliminated 5–4 on aggregate. In May 2015, Ainsworth scored in both legs as Motherwell defeated Rangers 6–1 on aggregate in the Scottish Premiership Play-off Final to retain their place in the top flight. Following an incident at the final whistle of the second leg involving Motherwell's Lee Erwin and Fraser Kerr and Rangers' Bilel Mohsni which saw Kerr and Mohsni given red cards, two days later Ainsworth along with teammate Craig Moore received a two-match ban for his part in the incident.