Grimes was born in Chertsey, Surrey, and then attended Kingston Art School, later studying in Paris. He joined the Army (underage) in 1915, spending two years on the Somme before transferring to the Royal Flying Corps. Grimes worked in advertising before he took up cartooning, with Douglas Motorbikes one of his main clients (and as a hobby raced at Brooklands hanging out of 'sidecars', which his son describes as 'more like planks'. He also did 'technical drawings' for Motor Sport, illustrating the latest developments before the photos were available. In 1927 he succeeded David Low as political cartoonist on The Star newspaper in 1927. He produced many posters for the LCC and the Ministry of Labour, and one of the 'keep mum' series.

Grimes did some posters for the LCC Evening classes which for a time adorned the then boarded up Eros statue in Piccadilly Circus, amongst other sites (along with newspaper adverts). In an article illustrated with Grime's early poster effort 'He's Gone to LCC Evening Classes', Advertiser's Weekly noted that unlike most large posters, where simplicity was usually advocated, Grimes used a lot of detail. Grimes, however, had kept the detail 'in the background where it would not distract attention'. Grimes worked as a cartoonist for The London Evening Star, starting the series "All My Own Work" in 1938 - a series which went on the reflect many of the preoccupations of wartime Britons. Advertiser's Weekly felt that newspaper cartoonists 'often make very good poster artists', potentially because 'they have trained themselves to eliminate unnecessary details'. Grimes also went to the front as an 'official' war artist during the Second World War.

In 1952 Grimes retired and went to Spain to paint.