[4] On the other hand, there is his double vocation: the first, as a brilliant footballer, and the second, as a clever civil engineer, who built Bilbao's Arenal Bridge and the Yesa Reservoir.

[7] His father was a French engineer who was a high official of the Company of Railways in Northern Spain, holding the post of head of traffic.

[7] The Petit brothers made themselves noticed during the 1914–15 season, and despite René's early age, he also earned himself a spot on the first team, making his debut for Madrid alongside Juan in a Central Regional Championship match against Sociedad Gimnástica on 15 November 1914, scoring his side's only goal in a 1–3 loss, and in doing so at the age of 15 years and 37 days, Petit became the youngest-ever scorer in the history of Real Madrid.

[9][7] By scoring in the final on 15 May, he became, at the age of 17 years, 7 months and 7 days, in one of the youngest players to do so, and since this was believed to have been his first goal in the Copa del Rey (he had already scored against Sevilla in the quarterfinals two months earlier), later reports wrongly credited this age when making the list for the youngest scorers in Madrid's history, which puts him in fourth, only behind Raúl, Alberto Rivera, and Antolín Ortega.

[10][5] In total, Petit scored 14 goals in 29 games for Madrid in the Copa del Rey, plus winning one domestic cup title and two regional championships.

[9][7] Despite having just won the Copa del Rey with Madrid, his Basque inclinations motivated Petit to leave the club as he preferred to continue playing football for the team of his hometown, the Real Unión de Irún, which had just been founded.

[7][8][5] As he was studying engineering in the capital, Petit would travel to his hometown by motorcycle on the weekends to wear the colors (also white) of the Real Unión.

[7] Despite his French links, Petit was incorporated to these preparatory matches, and he even played in one of them on 20 July, in San Mamés, helping the Probables to a 2–0 win.

In order to field Petit, Spain then sought authorization from the IOC, claiming that he had played his entire life, since he was fourteen years old, in the Spanish Championships, having held the title of Champion twice.

[7] Already during training, I could see that René was one of the best players of the moment, although the French selectors did not know his worth too much, nor did they trust him too much, since they even doubted a lot about whether or not they should select him, but then they were the first to marvel at his brilliant football.

During his stay in Antwerp, Petit, who spent a lot of time in the company of his Spanish teammates, was part of a negotiation between him, Pagaza, and Pentland, which convinced the latter to start coaching in Spain; this intermediation would greatly influence the future of Spanish football since Pentland went on to become the greatest coaches in Spain during his time.

[17] In 1924, Petit was forced to choose between representing France at the 1924 Summer Olympics and Real Unión and he chose his hometown team.

[6] Petit was one of the players who contributed the most to developing football fans in Spain, due to his beautiful short and mathematical passing style.

[8] I became a civil engineer in Madrid because I loved Nature, living in constant contact with it, and I thought that this career could fulfill my aspirations in that regard.

[7] In fact, after completing his studies at the Madrid School of Civil Engineering [es] and finishing his degree in 1925, Petit carried out several small public works and professional jobs in Gipuzkoa and Navarra.

[4] He was also in charge of the works on the Bardenas Canal and the irrigation of this area would be his greatest personal satisfaction, as he later declared that "The best payment I have ever had is seeing how authentic deserts have become green, beautiful and profitable".

[4] He collaborated with José and Ramón Iribarren in the "Plan for expansion and internal reform of the city", being in charge of establishing the sanitation network and channeling the Bidasoa.

[4] At the beginning of the Civil War, he had to briefly go into exile in France because he had showed sympathy for the uprising in Irún, which was in a republican zone.

[4] In 1945, under the direction of René Petit, two renovations of the Yesa Reservoir project were carried out, the first one was approved by Ministerial Order on 6 February, and the second and definitive one on 15 September.

At the end of these works, in 1959, he moved, as second chief, to the head of Public Works of Gipuzkoa,[4] but following the inauguration of the Yesa Reservoir by Francisco Franco in late 1959, Petit decided to settle down to spend more time with his family, thus accepting a position at the San Sebastián Highway Headquarters, where he remains until his retirement in 1969, always maintaining his domicile in Fuenterrabía.