[4] Petit was born in Irun, Gipuzkoa on 2 October 1896,[3][a] as the son of a Spanish mother from Madrid and a French engineer who was a high official of the Company of Railways in Northern Spain, holding the post of head of traffic.
[11] Even though his career was overshadowed by his brother René, who became a star of national football during the following decade, Juan was nonetheless a player of class and finesse, who was a magnificent midfielder, fine and with a phenomenal control of the ball, who formed a lethal left wing with Sotero Aranguren, as they had great understanding in their passes and combinations.
[4][13][14] In his last season at the club in 1916–17, Petit started the pre-season strongly with a leading role in the friendly against Fortuna de Vigo on 1 October, scoring a double and being described by the local press as having "a scientific game that was astonishing".
[4] Petit played his last official match for Madrid on 1 April 1917, scoring the opening goal in an eventual 4–1 win over FC Espanya at Campo de O'Donnell in the first leg of the 1917 Copa del Rey,[4] but shortly before the 1917 cup final, Petit was drafted for the French army during the First World War, while his brother René stayed in Madrid due to still being a minor of 17, going on to score a late equalizer in the final to help his side to a 2–1 win over Arenas de Getxo.
[4] Just as he had done at Madrid FC, he again shared the pitch with his brother René, helping Union to a 1–4 win over Real Sociedad at Atotxa Stadium, with Juan scoring the last goal.
[4] This victory qualified the club for the 1920 Copa del Rey, starting in the quarter-final tie against FC Barcelona, which then had the likes of Paulino Alcántara and Josep Samitier; Union lost 5–4 on aggregatte.
[4] Years later, an AS Color special on the 75th anniversary of Real Madrid included a profile describing Juan Petit's playing style as "A long-striding forward who passed wonderfully, and he finished hard with his left foot".
[4] In September, Petit was injured in France by a kick received from a horse, but it was a minor wound, and in the following month, he had a brief stay in Hendaya before returning to the front to join his artillery regiment.
[4] In September, he was convalescing from his injuries in Hendaya, where he was visited not only by the several Real Madrid members who spend their summers in the North of Spain near the border, but also by his girlfriend Solita who traveled to France with her uncles.
[4] On 8 May 1921, the 25-year-old Juan Petit married Soledad Iruretagoyena in Irún, with their honeymoon taking place in Guétary (France) and later they went on a tour of the French front, where he had bravely risked his life during the War.