In the books, there are many families of fieldmice that make the journey to White Deer Park including Fieldmouse, who is the elected leader of the group. The fieldmice travel for most of the journey with their cousins the voles. Along the way, several of the voles and fieldmice have babies and all the fieldmice decide to stay behind to look after them. However, the babies and some of the adults are killed by a shrike, so the voles and fieldmice decide to rejoin the group. The fieldmice are among the animals that are carried across the motorway by Whistler. They are also among the group that is left behind when the animals flee from the church, but they are all brought back to the main group by Whistler.
Once in White Deer Park, the fieldmice struggle to survive through the first winter and as a result, only three fieldmice are alive to see the spring. They are able to reproduce, but in an attack by Scarface in Fox's Feud, Fieldmouse and several of his family are killed. All of the original Farthing Wood fieldmice had died by the start of The Siege of White Deer Park, leaving behind their offspring that were born within the park.
It is assumed that the ancestors of the Farthing Wood fieldmice appear in Farthing Wood: The Adventure Begins, the prequel set five years before the journey to White Deer Park. Groups of mice are mentioned in the book, but it is not specified whether any of them are fieldmice. It is likely though that there were fieldmice living in the wood at this time, possibly along with other species of mice which may have died out before the journey to White Deer Park begins.
In the TV series, only two fieldmice make the journey to White Deer Park; Fieldmouse and Mrs Fieldmouse. However, in some scenes it appears that there are up to five or six fieldmice on the journey, but they always disappear soon after, suggesting that this is a canonical error. Mrs Fieldmouse has three babies on the journey, who are all killed by the shrike, but both adult mice survive the journey. Once in the park, Mrs Fieldmouse is killed and eaten by Kestrel, who does not recognise her as a Farthing Wood creature. Fieldmouse survives through the first winter and is seen with other animals in the final episode of the second series. However, he does not appear in the third series, there were some fieldmice but no evidence that there Farthing Wood or White Deer Park.