After retiring, he began coaching the Warringah Dolphins in the New South Wales state leagues, before coaching
NSL side Newcastle Breakers, followed by a stint at the
Brisbane Strikers. Both stints were largely unsuccessful, his sides reaching the finals only once.
Adelaide United In 2003, he became the inaugural coach of
Adelaide United. Kosmina coached United to an impressive 3rd in the final season of the NSL. Maintaining his job for the inaugural season of the
A-League, Kosmina led his side to the Minor Premiership but a poor finals series saw them finish 3rd overall. The next season saw mixed results for Kosmina and Adelaide United. Finishing 2nd on the table behind
Melbourne Victory, Adelaide managed to reach the grand final, only to be thrashed 6–0. Kosmina's criticism of the refereeing of that match, coupled with the devastating loss and a touchline ban earlier during the season for a
scuffle with Victory captain
Kevin Muscat, saw him forced to resign by the Adelaide board. Kosmina said that he felt that Muscat had thrown an elbow at him and said he had "lost it". Since signing with
Sydney FC Kosmina made an immediate impact; winning his first game with Sydney 3–2 against rivals
Central Coast Mariners at the
Sydney Football Stadium. Sydney FC beat the
LA Galaxy 5–3 at Sydney's
Telstra Stadium in a friendly, and also won a thrilling 5–4 victory against the
Central Coast Mariners at
Bluetongue Stadium. Following Sydney FC's poor run in the 2008/2009 season (missing the finals for the first time), John Kosmina had his contract terminated in late January 2009.
Adelaide Raiders He was announced the manager of the
Adelaide Raiders, a semi-professional club playing in the
FFSA Super League. On 1 September 2011, it was announced he had signed as senior coach of
FFSA Super League club
Croydon Kings after more than 35 years away from the club. It was announced on 18 December 2011 that Croydon Kings had agreed to release Kosmina from his coaching contract so he could accept the Adelaide United manager position.
Return to Adelaide United On 18 December 2011 he signed as caretaker coach with
Adelaide United for the remainder of the 2011–12 A-League season. On 22 March 2012 it was announced he had signed a one-year contract with the club to stay on for the 2012–13 A-League season. On 28 January 2013 he stood down as manager of Adelaide United, citing a lack of trust at the club.
Brisbane City In August 2015, Kosmina was appointed senior coach and football leader at
National Premier Leagues Queensland club
Brisbane City, signing a three-year contract. In November 2018, it was announced that he would join the coaching staff at
St Joseph's College, Gregory Terrace, taking responsibility for the Open First and Second XI teams, as well as the Year 10 and 11 teams, as part of a partnership between the college and Brisbane City.
Brisbane Strikers In November 2020,
Brisbane Strikers announced that Kosmina would return to the club as senior coach, 17 years after he last coached the side. After a string of shocking results Kosmina was sacked by the club on Monday 24 May On 24 May 2021, it was announced via a club statement on the club website that the
Brisbane Strikers and Kosmina would be parting ways. This decision came after a poor run of form which saw the Strikers not take a single point from their first ten games in the
2021 NPL Queensland Season. ==Media==