Longmire's physique and size earned him the nicknameHorse. At North Melbourne, he formed a powerful goalkicking partnership with centre half forwardWayne Carey. In six seasons between 1990 to 1995 Carey and Longmire collectively kicked 768 goals (of which Longmire contributed 464) and 13 times they combined for 10 goals or more in a game. Individually, Longmire kicked 5-plus goals in a game 36 times, 7-plus goals 18 times and 10-plus twice, before a serious knee injury force him out of the game for the 1996 season. When he returned the following year, he played out the rest of his career in defense and in the ruck.
Longmire is best known for his performances during the 1990 season. At only 19 years of age he kicked 98 goals and won the Coleman Medal as the league's leading goal kicker. In round 2 of that year he kicked a North Melbourne record, 12 goals against Richmond, which he broke twelve weeks later when he kicked 14 goals in round 14 against Melbourne. Going into the final round Longmire looked certain to reach the 100 goal milestone for the season, however terribly inaccurate kicking resulted in a tally of 2 goals and 8 behinds, leaving him just 2 goals short of the milestone. Longmire went on the win North Melbourne's best and fairest that year and led the club's goal kicking list each year from 1990 to 1994.
Longmire's last career game was the 1999 Grand Final, in which the Kangaroos defeated Carlton.
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's first game as the Sydney Swans coach ended in a draw against Melbourne, with both teams scoring 11.18 (84). His first win as coach came the next week, against Essendon in Round 2. Longmire had a relatively good start to his coaching career, with only five losses in the first fourteen rounds of the season (albeit against top-four opposition in Geelong, Carlton (twice), Hawthorn and Collingwood).
One of his best coaching achievements was engineering Sydney's upset 13-point victory over Geelong at Skilled Stadium in the penultimate round of the 2011 season. The Swans had not won there in more than 12 years and the home team had not lost at the ground in exactly four years and one day. Also, the Swans were the only team to beat top-four side West Coast at Patersons Stadium during the season. Those two sides won the rest of their home matches during the regular 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.
Semi-Final: South Australia 12.8 (80) defeated New South Wales 8.11 (59), at Football Park, 3 March 1988 3rd Play-Off: New South Wales 10.8 (68) defeated Western Australia 9.12 (66), at Football Park, 5 March 1988