A fractured tibia suffered after just 15 minutes during a 1–0 win away to Plymouth Argyle on 19 March 2016 ended Mackail-Smith's season, having made 34 appearances, scoring four goals in all competitions. Mackail-Smith suffered a further setback in pre-season ahead of 2016–17 when he underwent surgery to insert a pin in his fractured tibia, ruling him out for another six weeks. However, he made a full recovery and made his return as a 70th-minute substitute for Jordan Cook in a 3–1 win away to Exeter City in the FA Cup on 5 November 2016. Mackail-Smith struggled to force his way into the first-team and returned to League One club Peterborough United on loan until the end of 2016–17. His first appearance after returning on loan came four days later in a 3–0 victory away to Port Vale and completed the loan spell with 18 appearances and five goals. Mackail-Smith was released when his contract expired at the end of 2016–17. On 17 August 2017, Mackail-Smith signed for League Two club Wycombe Wanderers on a one-year contract. He signed a new two-year contract with Wycombe on 26 April 2018. Mackail-Smith joined League Two club Notts County on 30 January 2019 on loan until the end of 2018–19. He joined another League Two club, Stevenage, on 2 September 2019 on loan until 12 January 2020. His first goal for Stevenage was the winning goal in a 1–0 victory over Bristol Rovers in the EFL Trophy third round on 8 January 2020, which set up a quarter-final tie against Exeter City. The loan was extended until the end of the season on 17 January. While with Dagenham & Redbridge, Mackail-Smith received his first international recognition being called up to play for the England C team, winning seven caps and scoring four goals. He helped England C win the 2005–06 European Challenge Trophy, scoring twice. Mackail-Smith qualified to play for Scotland through his Edinburgh-born grandmother Margaret and confirmed his interest in playing for Scotland if called up in May 2009. On 24 March 2011, he received his first senior international call-up from Scotland boss Craig Levein for a friendly with Brazil on 27 March. He made his international debut when he came on as an 87th-minute substitute for Kenny Miller. He scored his first international goal against Liechtenstein on 8 October in a crucial Euro 2012 qualifier, and played in the match against Spain in Alicante on 11 October, winning a penalty from which David Goodwillie scored, in a match that Spain won 3–1.