Deans attended Christ's College, Christchurch as a boarder where he played rugby mainly at first five-eighth.

Deans was appointed as coach of the Canterbury team for the 1997 National Provincial Championship (NPC) season.

Deans' first season was a success, with Canterbury beating Auckland for the first time in 14 years, then going on to win the title.

A victory would have seen Deans coach teams to Super 12, Ranfurly Shield and NPC titles in the same year; however eventually Wellington won 34–29.

In 2008, Deans' last season in charge of the Crusaders, he coached them to a seventh super rugby title after they beat the NSW Waratahs at Jade Stadium.

Following 22 February earthquake, there remains doubt as to whether AMI Stadium will ever be used again, after some of the ground, including parts of the Deans stand, suffered structural damage.

The contract was for four years – leading into the 2011 Rugby World Cup – as Deans became the first ever foreigner to coach Australia.

[10] Shortly after the details were made official, Deans released a personal statement on the Crusader's official website:[11] [...] The decision to leave Christchurch after the end of the 2008 Crusaders season and coach overseas has undoubtedly been one of the most difficult of my professional sporting career.

After serving for eleven years as a Super rugby manager and coach here, I feel I can not afford to forgo the position offered to me in Australia.

Not only do we have a good level of depth among our player base, there are some quality people in place around the team, and some highly promising coaches coming through.

In 2010 Deans also became the first Wallabies coach to win on the South African highveldt in 47 years after the team won 41–39 in Bloemfontein, thanks to a penalty goal from Kurtley Beale with no time left.

While coaching the Wallabies in 2011 they had a mixed year - losing to Samoa before another loss to the All Blacks in Auckland, and defeating South Africa both home and away.

[12][nb 1] Dissatisfaction amongst players and supporters with the conservative game plan employed by the Wallabies during and after the World Cup, a win rate hovering around 60%, and a poor 3–15 record against the All Blacks led to Deans came under increasing pressure to keep his coaching position.

[16][17] During his six-year tenure, Deans coached the Wallabies on 74 occasions, winning 43 times, losing 29 and drawing twice.

He holds a good record against the Springboks, 9 wins from 14, and led the Wallabies to a Tri Nations championship in 2011 and to 3rd place in the 2011 Rugby World Cup.

On 21 April 2014, Deans was announced as the Panasonic Wild Knights new head coach, succeeding Norifumi Nakajima.

[19] On 23 June 2017 it was announced that Deans is to coach the Barbarians FC for their match against New Zealand at Twickenham on 4 November 2017[20] His brother, Bruce, was an All Black and member of the Canterbury team.