Soon he replaced his father in that place, due to his talent and due to his father's death when Seixas was 14. Neverthless he didn't occupied that place for a long time and in 1720 he moved to Lisbon where

He developed his capabilities .He was an excellent key-performer and composer.He was named organist in the Lisbon Cathedral, which gave him also functions in the Royal Chapel.At the same time he becomes a musical teacher of the most noble families.

Domenico Scarlatti who came to Portugal in 1721 contracted as 1st master of the Royal Chapel and teacher of the infant D.Maria Barbara, considered Seixas one of the best teachers he had ever heard.Even though he had contact with a napolitan master, Seixas never let himself dominated by the italianism that ruled the music world at that time.

He always proved his originality.His successful career gave him the privilege to become a member of the Sant'Iago Order, the title of captain of the ordinance of the court and a knight of the Christ's Order.

He was a plebeian, and the position he acquired proves again his talent.He married in 1732 and had 2 sons and 3 daughters.

He was so famous that when he died, the eremite monks of Saint "Franciso"of the Graca Convent dedicated him solemn exequies to which the folk and the Court nobility presenced as well.