[2][3] Gastón Esmerado promoted the defender into the club's first-team squad during the 2017–18 Primera División campaign, making appearances against Newell's Old Boys, Chacarita Juniors and Patronato from February 2018 as they were relegated; his only previous experience in senior football was as an unused substitute in the Copa Argentina versus Defensa y Justicia in August 2017.

[2][4] In Primera B Nacional, the second tier, Demartini started and finished the opening twenty fixtures of 2018–19, while also netting his first goal during a 2–3 home defeat to Arsenal de Sarandí on 23 September 2018.

[2][5] On 15 March 2021, Demartini joined Chilean Primera División club Antofagasta on a loan deal for the rest of 2021.

[6] In January 2022, he was loaned out once again, this time to Argentine Primera División club Tigre until the end of the year.

[7] In 2018, Demartini represented Argentina's U19s at the South American Games in Bolivia; as they finished fourth.