He played in the English Football League for Brighton & Hove Albion, Luton Town, Peterborough United, Wycombe Wanderers, Notts County and Stevenage, and at senior international level for the Scotland national team.

[8] Mackail-Smith was loaned out to Isthmian League Division One North club Arlesey Town in 2002–03, before making a permanent move for the 2003–04 season.

[9] He made 54 appearances, scoring 17 goals during the season,[10] including a brace in a 4–2 win at home to Dagenham & Redbridge in the FA Trophy on 3 February 2004.

[12] On 29 January 2007, Mackail-Smith was signed by League Two club Peterborough United, with Dagenham retaining 15% of any future transfer fee.

He described him as "a rampaging Nordic Viking with his long blonde hair" and was impressed by the way he would pick up the ball and run at defenders, attacking at every opportunity in what Claridge described as a positive way.

[41] Mackail-Smith returned for Brighton's final run-in to the 2013–14 season, playing in both legs of the play-off semi-final against Derby County, which the club lost 6–2 on aggregate.

[42][43] During the 2014–15 season, Mackail-Smith found himself back in the first-team under new manager Sami Hyypiä, but struggled to score, netting just three goals in 34 appearances in all competitions.

[45] Mackail-Smith rejoined Peterborough United on loan on 27 November 2014, a move that saw him link-up once more with striker Aaron McLean, having scored 183 goals between them during their previous partnership at the club from 2007 and 2011, which included three promotions.

[48] In July 2015, Mackail-Smith began training with League Two club Luton Town, headed up by his former Dagenham & Redbridge manager John Still.

[49] He featured in numerous pre-season friendlies, rebuffing interest from League One teams Blackpool and Southend United,[50][51] before signing a two-year contract with Luton on 1 August.

[55] 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.

[56][57] 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.

[69] Mackail-Smith was part of the Bedford Town side which was crowned Southern League Division One Central Champions in the 2021–22 season.

[71] 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.