Longmire began his playing career at the Corowa-Rutherglen club in New South Wales, where he nearly won the Ovens & Murray Football League’s leading goalkicker title in 1987 as a 16-year-old, kicking 82 goals. His ability and size quickly attracted the interest of the North Melbourne VFL club’s talent scouts.

Longmire’s physique and size earned him the nickname Horse. His first match for North Melbourne was in the infamous Exhibition Match between North Melbourne and Carlton at The Oval in London in 1987. His first official match was in in 1988 against Footscray with a four-goal performance from full-forward, but struggled after that and near the end of the season coach John Kennedy Sr. moved him to full-back. He did well in that role during 1989 – holding Tony Lockett to five kicks in Round 14 – but North’s lack of key position players in attack saw him moved back to the forward line in August.

1990 saw Longmire jump to the top of the tree: at only nineteen years of age, he kicked 98 goals and won the Coleman Medal as the league’s leading goal kicker (and the youngest player to have done so). In Round 2 of that year he kicked a North Melbourne record of twelve goals against Richmond, which he broke twelve weeks later when he kicked fourteen goals in round 14 against Melbourne. Going into the final round Longmire looked likely to reach the 100-goal milestone for the season, however terribly inaccurate kicking against a hard Collingwood defence resulted in a tally of two goals and eight behinds, leaving him just two goals short. Longmire went on the win North Melbourne’s best and fairest that year and led the club’s goal kicking list each season from 1990 to 1994.

Longmire returned to New South Wales to take up an assistant coaching position with the Sydney Swans. In 2006, he was considered to be a front-runner for the St Kilda Football Club coaching role, which was made vacant by the sacking of Grant Thomas, however, the role later went to then-fellow Swans assistant coach Ross Lyon. In 2008 the coach, Paul Roos, appointed Longmire the Swans’ “coaching co-ordinator”. Longmire replaced Roos following his retirement at the end of the 2010 season.

Longmire took Sydney to the finals in 2011, his first year as senior coach in what was the club’s 13th finals appearance in 16 seasons. After beating St Kilda in the elimination finals at Etihad Stadium, the Swans were defeated by Hawthorn in the semi-finals ending what was otherwise a promising first season for Longmire in the top job.

In 2012, his second year as coach, Longmire led Sydney to third place on the AFL ladder, compiling an impressive 16–6 record over the home-and-away season. He later coached the team to a 14.7 (91) to 11.15 (81) victory over Hawthorn in the 2012 AFL Grand Final. Subsequently, his contract was extended until the end of the 2015 season.

In March 2014, Longmire signed a two-year contract extension that takes his tenure to at least the end of the 2017 AFL season.

In 2020, Longmire coached the All-Stars team in the one-off 2020 State of Origin match that was played on 28 February 2020 at Marvel Stadium.