Born in São Miguel Island, Azores, Diniz and his family emigranted to the United States and settled in New Bedford, Massachusetts.

Diniz served in the United States Army during World War I and was awarded the Purple Heart.

He was in the furniture and insurance business and also served a deputy sheriff for Bristol County, Massachusetts.

During his tenure in the House, Diniz was twice ejected for controversial remarks, but was allowed to return after apologizing.

In 1947 and 1948 he was Democratic nominee for the Massachusetts's 9th congressional district seat, but lost to Donald W. Nicholson.