He previously coached St Helens in the Super League, the Gold Coast Titans in the NRL, and Papua New Guinea.

He worked on the coaching staff of the Canterbury-Bankstown Bulldogs, St George Illawarra and Parramatta before joining the Sydney Roosters as right-hand man to Trent Robinson.

Holbrook then coached St Helens to their 14th championship win after defeating Salford 23–6 in the 2019 Super League Grand Final at Old Trafford.

In round 4 against the Wests Tigers, Holbrook guided the club to their first win in 364 days as they won the match 28–23 at Suncorp Stadium.

Assistant Coach Jim Lenihan is due to take over for the rest of the season beginning with their Round 17 match against the Brisbane Broncos before Hasler joins[14][15] Following his departure from the Gold Coast club, Holbrook spoke with the media saying "I did not see this coming at all, that’s what I am pissed off about, I could understand if they said, 'Look you have three games or whatever time period to do something', but to be moved on like this, I’m still coming to terms with it".

[16] On 24 August 2023, it was reported that Holbrook had taken a deal to coach the Papua New Guinea national rugby league team for the end of season pacific championship.