Not "master worker", which is nothing more than a literal translation used by Spanish-speaking authors (in the cases Adolfo has quoted), who don't seem to be aware of English terminology in this area. "Master worker" doesn't really mean anything. A master builder (maestro alarife), in the medieval and early modern guild system, is a builder who has attained the status of master craftsman. But in this context that's not what it means.

This term is tricky, because the "Obrero Mayor" of a cathedral, in Spanish church terminology, is not a builder or an architect at all, but an administrative position: it is a member of the cathedral chapter (that is, a canon), responsible for organising and overseeing the construction and repair works required in the fabric of the cathedral, deciding what works are required, and paying for them of behalf of the chapter. In English we usually call this position "the Keeper of the Works". The position still exists today. Here's a list of the chapter of Toledo cathedral in Spain, and as you can see the "Obrero Mayor" is a canon, "D. Antonio Cabrera Delgado y Silveira":
http://www.catedralprimada.es/cabildo_miembros/

The Keeper of the Fabric of Bristol Cathedral in England, who performs the same role, is currently Canon John Cannon (great name!):
http://bristol-cathedral.co.uk/about-us/who-we-are/

On this sort of historical terminology we need to use old dictionaries to understand what the terms really mean. The relevant meaning of "obrero", which does basically mean "worker", is this (from the first RAE dictionary, known as the Diccionario de autoridades, of 1726-39):

"Se llama tambien el que cuida de las obras, en las Iglesias o Comunidades, que en algunas Cathedrales es dignidad. Lat Fabricae praefectus, vel administrator. [...] el Cabildo nombró por obréro al Canónigo Juan de Contreras".

BUT this is all for the record; it doesn't apply in this case, because the term "Obrero Mayor" is being used loosely here and it really means "Maestro Mayor". Melchor Pérez de Soto was an architect, not a cleric or administrator. But he was not a worker or "builder" either.

"La biblioteca que poseía en México el arquitecto Melchor Pérez de Soto"
https://books.google.es/books?id=jcT3IbPIoBAC&pg=PA94&lpg=PA...

"Melchor Pérez de Soto, maestro mayor de la catedral"
http://campaners.com/php/textos.php?text=13

This (mis)use of the term "obrero mayor" was not unusual in New Spain. Pérez de Soto is indeed referred to as obrero mayor in original documents, and there are other examples, discussed in detail in this article, which includes the following comments:

"El mismo tomo 112 del Ramo de Historia llama a los Dávila "obreros mayores" o sea personas encargadas de la administración económica o material de las construcciones. Esto naturalmente no excluye la posibilidad de que un arquitecto ocupase también el puesto de obrero mayor y entonces es aún más difícil desentrañar las pocas notas alusivas a constructores de la época colonial; el término "obrero mayor" resulta pues algo ambiguo. [...]
Creo que en este caso "obrero mayor" debe leerse también como 'maestro mayor" de manera que este puesto también vacaba durante cuatro años."
http://www.analesiie.unam.mx/pdf/11_19-39.pdf

So I think we should translate this as if it said "maestro mayor", which is what it means. The Maestro Mayor of a Cathedral was the Master of Works, and that is what he actually was:

"Sir James Murray of Kilbaberton, (d.1634), was a Scottish master wright and architect. He served as the King's Master of Works under James VI"
https://en.wikipedia.org/wiki/James_Murray_(architect)

As a municipal post "maestro mayor" is normally called a city architect, but in the context of a cathedral "master of works" is more usual. But effectively it is an architect, and that's really what Pérez de Soto was.

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Note added at 4 hrs (2015-09-26 08:34:41 GMT)
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Of course, if it is actually the case that Pérez de Soto, as well as being master of works, occupied the administrative position properly called "obrero mayor" in the cathedral chapter (which was sometime a lay canon), then we could call him keeper of the works. To determine this would require research I'm not in a position to do. But for now, I'd play safe and translate this as "master of works".