Shortly after making his debut for Badalona in 2011, Mariano joined Real Madrid, where he played in the C-team and then the reserves, where he was top scorer in the 2015–16 Segunda División B.

He then began playing with the first team, and was part of their squad that won La Liga, the UEFA Champions League and the FIFA Club World Cup in 2016–17.

On the 31st, he made his first start, playing the entirety of a 3–1 loss at RB Linense in that season's Copa del Rey, scoring his team's consolation.

On 29 March, he entered late in De Tomás' place again and scored twice to cap a 5–1 home win over UD Las Palmas B,[8] while one week later, he was given a start at lowly UB Conquense and struck both goals in a seven-minute span.

[13] Mariano was definitively promoted to the main squad by manager Zinedine Zidane on 20 August 2016, after a back injury to striker Karim Benzema.

[14] One week later, Mariano made his senior debut during a 2–1 win over Celta de Vigo, replacing Álvaro Morata in the 77th minute.

[17] Mariano scored his first top-flight goal on 10 December, equalising in a 3–2 comeback victory at home to Deportivo de La Coruña.

[18] Later that month, he was part of the squad that won the 2016 FIFA Club World Cup in Japan, but did not appear in either of Real Madrid's matches.

[32][33] His second debut was on 19 September, coming on as a 73rd-minute substitute against Roma in the first fixture of the 2018–19 UEFA Champions League group stage, and he scored the last goal of a 3–0 victory with a curved shot from outside the penalty area.