John V was born on 24 December 1389 at the Château de l'Hermine as the eldest son of John IV, Duke of Brittany and Joan of Navarre. He became Duke of Brittany in 1399 when he was still a minor upon the death of his father. His mother served as regent in the initial portion of his reign.
Unlike his father, John V inherited the duchy in peace, as the end of the Breton War of Succession and John IV's military conquests in Brittany promised. However, his father's rivals for the duchy, the Pentheiveres, continued to plot against him. Furthermore, John had to secure the peace of the duchy during an unstable period culminating in King Henry V of England's invasion of France.
He became duke at the age of ten, and began his reign under the tutelage of Duke of Burgundy, Philip the Bold, who was ravaging nearby Jersey and Guernsey. He made peace with the king of France, Charles VI, whose daughter, Joan of France, he married. He also reconciled with the powerful magnate Olivier de Clisson, formerly an enemy of his father. In 1404, he defeated a French force near Brest. A potential conflict with Clisson was averted by the latter's death.
When Henry V invaded France, John was initially allied to the French. However, he missed the Battle of Agincourt. His brother Arthur de Richemont participated, though, and was captured and imprisoned by the English. The confusion in the aftermath of the battle allowed John to seize Saint-Malo which had been annexed by the French. He then adopted a policy of switching between the two parties, English and French. He signed the Treaty of Troyes, which made Henry V heir to France, but he allowed his brother Arthur de Richemont to fight for the French.
The Counts of Penthièvre had lost the Breton War of Succession (1341–1364) in which they had claimed the ducal title of Brittany from John's grandfather, John of Montfort. The war ended in 1364 in a military victory for John's father, in which the Penthièvre claimant, Charles of Blois, died. His widow, Joanna, Countess of Penthièvre, was forced to sign the Treaty of Guérande which concluded the conflict. The treaty stated that Penthièvres accepted the Montforts's right to the dukedom, but if they failed to produce a male heir the duchy would revert to the Penthièvres.