After a mooted move to Sevilla collapsed, Mariano completed his return to Madrid on Friday (AEST) after spending a single season in Ligue 1 with Lyon.
Mariano is the only major attacking signing made by Madrid after Ronaldo's shock departure to Juventus in July.
Club icons such as Emilio Butragueno and Raul have also worn seven and Mariano is hoping to prove himself worthy of the shirt.
"For me the most important thing is not the number," Mariano said, despite reports claiming he asked to succeed Ronaldo.
"It is a challenge and a pride to carry the same number as all the legends, like Emilio Butragueno, and the great players who have carried it.
"I was excited, I was free and I am very happy to wear the seven."
Sevilla hit out at Mariano for rejecting a move to the club in favour of returning to the capital, but the striker wants to focus on his future under Julen Lopetegui.
"Regarding Sevilla I only have words of thanks for the interest they have shown for me," Mariano added. "Today is the best day of my life to sign for the best club in the world.
"Julen called me and is responsible for my signing. He told me that he was counting on me and that he would be very happy if he decided to return.
"When you leave your house, you leave sad. It's hard to imagine coming back after a year and when I received the call Julen I was happy and nervous."