América Lainez was recruited by scout Ángel González along with his brother
Mauro. Mauro decided to join
Pachuca's academy, while Diego opted to join
Club América. On 1 March 2017 Lainez made his competitive debut with América in the
Copa MX group stage match against
Santos Laguna. Three days later, he made his
league debut against
León, becoming one of the club's youngest players to debut for the first team at 16 years old. On 19 March, after a lineup error that ruled out
Gerson Torres from being eligible to play, Lainez was assigned to start against
Pumas UNAM. On 4 August 2018 Lainez scored his first two goals for América in a 3–1 win over
Pachuca at
Estadio Hidalgo; he became the youngest player to score a double with the club, and the fifth youngest to score a goal in the club's history. On 18 September he suffered a sprained ankle on his left foot during training following a tackle from
Bruno Valdez, forcing him to miss out on four weeks of action, including the
Súper Clásico against arch-rivals
Guadalajara. He would return from injury on 20 October, and scored his first goal playing in the
Estadio Azteca as América defeated
Tijuana 3–0. Lainez scored in both legs of the
Apertura semifinal series against Universidad Nacional, making him the youngest player to ever score during the playoff stage for the club.
Betis On 10 January 2019 Lainez joined
La Liga club
Real Betis on a five-year contract reportedly worth $14 million. The move made him the youngest player to emigrate from the Liga MX, as well as the second-most expensive transfer paid for a Mexican player by a European club. He was handed the squad number 22. He made his debut in La Liga on 20 January in a 3–2 home victory over
Girona, making his appearance as an 81st-minute substitute. The following week, Lainez earned a starting berth and played 75 minutes in Real's 1–0 defeat to
Athletic de Bilbao. On 14 February he became the youngest Mexican player to score in the
UEFA Europa League when he netted an injury-time equalizer in a 3–3 draw with
Rennes in the first leg of the round of 32. On 19 December Lainez scored his second competitive goal for Betisa free kickin the 4–0 victory over
Antoniano in the
first round of the
Copa del Rey.
Loan to Braga On 29 July 2022 Lainez joined
Primeira Liga club
Braga on a season-long loan. On 12 August he made his debut during a 3–0 win over
Famalicão, entering as a substitute at the 73rd minute. On 28 August he scored his first and only goal with Braga in a 6–0 win over
Arouca. After failing to make an impact at the club, his loan deal was terminated early.
UANL On 30 January 2023, Lainez returned to Mexico to join
UANL on a one-year loan.{{cite web|url=https://www.realbetisbalompie.es/noticias/actualidad/el-real-betis-y-el-club-tigres-uanl-llegan-a-un-acuerdo-para-la-cesion-de-lainez-28652|title=El Real Betis y el Club Tigres UANL llegan a un acuerdo para la cesión de Lainez ==International career==