Since I don't want this post to be too lengthy, I suggest that readers interested in details regarding Sheets' life and career link to here and here for plenty of information. His son maintains this site, the page I linked to containing examples of works that are confirmed as not being Sheets' paintings and others of dubious provenance.
Below are examples of what Sheets did.
This early painting is perhaps his best-known. It deals with what might be the station platform for a funicular that transported people to the now vanished Bunker Hill neighborhood in downtown Los Angeles.
This also looks like the old Bunker Hill area.
Details regarding this project can be found here.
Sheets was not a licensed architect. But his Millard Sheets Designs Company did have architects working under his direction. Sheets was responsible for many (most?) of the distinctive Home Savings buildings with sculpted motifs that graced California from the 1960s into the 1990s. Home Savings is no more, having been passed to Washington Mutual and then Chase. Many of the former Home Savings buildings have lost their former distinctiveness.