Mice were created by Aslan soon after the creation of the Narnian world, but were not originally chosen for the gift of speech.
In the Narnian Year 1000, after Aslan was killed by the White Witch, mice chewed through the ropes in which he was bound. These mice and their descendants became talking mice as a reward for this kindness.
A troupe of twelve talking mice, led by Reepicheep, participated in the war to drive the Telmarines out of Narnia, most notably in the Second Battle of Beruna. Since they could not reach high enough to kill their enemies, they stabbed them in the feet, causing them to hop around in pain, and often to fall. If they fell, the mice finished them off. If not, someone else did.
Some mice, during the time of King Tirian, also risked their lives to help the king when he was tied up and awaiting "Tashlan's" judgement during the last days of Narnia. They fed Tirian and gave him a drink, staying true to their noble consciences, despite their fear of Shift and Tashlan.