An accomplished virtuoso of both the organ and the harpsichord, Seixas succeeded his father as the organist for Coimbra Cathedral at the age of fourteen.

In 1720, he departed for the capital, Lisbon, where he was to serve as the organist for the royal chapel, one of the highest offices for a musician in Portugal, a position which earned him a knighthood.

[2] Citing his elegance and agility on the keyboard, he was a favorite teacher of many noble families, including the family of Luís Xavier Furtado de Mendonça [pt], the Viscount of Barbacena, where he gave harpsichord lessons to the Viscount's wife and daughters in exchange for artistic patronage.

[3] In Lisbon, Seixas met Italian composer Domenico Scarlatti, who was working in Portugal from 1719 to 1728 as appointed director of the court cathedral.

In an account by José Mazza in his Diccionario biographico de Musicos portugueses e noticia das suas composições of 1780, the king's brother, Dom António, arranged for Scarlatti to give Seixas harpsichord lessons.