Kilcock, County Kildare born centre half Mick O’Brien started playing football in the north east of England with junior clubs Walker Celtic and Wallsend in 1912, playing for Blyth Spartans and Newcastle East End the following year before joining Celtic in 1914. War intervened without O’Brien playing a senior match for The Hoops and he briefly joined Southern League Brentford later in 1914, resuming with the Griffin Park club in 1919.
That August he joined Southern League Norwich City before returning to the north east to play for Second Division South Shields, for whom he made his Football League debut at Hull City in January 1920, making only 3 appearances before joining Queen’s Park Rangers in May 1920, and he missed only 6 games in their inaugural Football League season in 1920-21, making his debut in their first Football League fixture that August against Watford.
He won the first of 10 Ireland international caps in February 1921 against Scotland, his last cap coming in April 1927 against Wales. He was signed by Second Division Leicester City in March 1922 for whom he scored 6 times in 67 games, before a move to Hull City in the summer of 1924, where he was made club captain. At Hull he scored once in 80 appearances before briefly joining Brooklyn Wanderers in the fledgling American Soccer League in May 1926, before returning to sign for Derby County that December. However he found opportunities limited at Derby and after only 5 games he joined Walsall in June 1928.
In 1927 he opted to represent the Irish Free State and won 4 additional caps between 1927 and 1932. A single season at Fellows Park saw 37 appearances, before he returned to Norwich City, by now a League club, in the summer of 1929, scoring 5 times in 65 appearances over 2 seasons. He moved finally to Watford in June 1931 scoring 6 times in 72 appearances before retiring from playing in 1933. After a spell as Brentford’s assistant manager he became Queen’s Park Rangers manager in May 1933 for 2 seasons in Division 3 (South), before becoming manager of Ipswich Town in May 1936 for a single season, leaving in August 1937.