Farmer got his start in the UK comics industry before becoming part of the British Invasion, the wave of UK creators that were an integral part of the DC Comics "new look" of the 1980s
He is primarily known these days as an inker, although he has done some pencilling as well (for instance, collaborating with writer Len Wein on an early 1980s run on Green Lantern). Like nearly all those involved in the British Invasion, Farmer got his start at the British weekly comic 2000 AD, where he pencilled such series as Judge Dredd and Anderson: Psi Division.
Farmer usually forms a team with writer/penciler Alan Davis. The pair, who first teamed on 1987's D.R. and Quinch for 2000 AD, have worked together on such titles as Marvel Comics' Excalibur, Avengers, Fantastic Four and Uncanny X-Men. For DC their work includes Superman and JLA as well as cover work on Green Lantern. He also helped co-write Gen¹³: Bootleg.
In 2004, Farmer inked John Byrne's pencils on Superman: True Brit