Monkstown, County Antrim born right back Alex Mackie started out at Monkstown and was playing with Forth River in Belfast in 1921, before being spotted by First Division Arsenal for whom he signed in February 1922. Mackie made his Football League debut against Birmingham City in December 1922. It was a rapid rise for the youngster, who had gone from third team right back to displacing England international right back Frank Bradshaw from the first eleven in just ten months. With Mackie and fellow Irishman Andy Kennedy installed as first choice full backs, Bradshaw never featured for the Gunners again.
Mackie soon became a regular in the Arsenal side and also made his international debut for Ireland alongside Kennedy, against Wales in a 3-0 victory at Wrexham on 14th April 1923. However, he was injured early on in 1924-25 and missed much of that season. Although Mackie would return to first team football, managing 35 League appearances in 1925-26, future Arsenal skipper Tom Parker joined in the spring of 1926 and soon ousted Mackie from the side altogether. Mackie played two more seasons for Arsenal in the reserves without making a single first team appearance, before leaving for Portsmouth in the 1928 close season. In all he made 118 appearances for The Gunners, scoring once.
At Portsmouth, Mackie became a stalwart for the next seven seasons. He made 286 appearances for Pompey, scoring twice, missing only 4 games in four seasons between 1930 and 1934, and he played in two FA Cup Finals at Wembley – against Bolton Wanderers in 1929 and Manchester City in 1934 – albeit Portsmouth ended up losing both. He also won two more caps for Ireland, playing in a 2-1 victory over Scotland at Windsor Park, Belfast, in October 1934 (over 11 years after his first cap) and in a 3-1 defeat to Wales at The Racecourse Ground, Wrexham in March 1935. He left Portsmouth in the 1935 close season, and finished his career with 19 appearances for Northampton Town over the next two seasons, and finally joined non league Sittingbourne in 1937 before his eventual retirement.