I am working on the bibliography of my Master's thesis with very particular formatting requirements, most of which I have managed with some help from here.
The last thing remaining for me is to use a comma after thesis type for Master's and PhD theses. This is what I've got currently:

What I want is:
PhD diss., The University...
Master's thesis, The University...
The solution presented here does not work for me; I get the exact same output.
Here is my code:
% Produce modified CMS citations and bibliography using bastardized MLA style
\usepackage[
citestyle=numeric, % force numeric inline citations
bibstyle=mla, % vanilla mla
natbib=true, % not sure why this is required
backend=biber, % compiling stuff
maxnames=7, % DTM guideline is up to 10
minnames=7, % biblatex defaults to minnames when max is exceeded, but need 7 per DTM
showmedium=false, % don't print publication medium
sorting=nty, % sort name, title, year
language=american % proper comma inside quotation behavior
]{biblatex}
\addbibresource{thbib.bib}
\DeclareFieldFormat{journalvolume}{#1} % Remove italicized formatting from journal volumes
\DeclareFieldFormat{booktitle}{#1} % Remove italicized formatting from journal volumes
\defbibheading{bibliography}{% % Change "Works Cited" to "Bibliography"
\section*{Bibliography}}
% \DeclareFieldFormat[thesis]{type}{#1\addcomma}
\DefineBibliographyStrings{american}{phdthesis = {PhD diss}}
\DefineBibliographyStrings{american}{mathesis = {Master's thesis}}
If I try adding the line \DeclareFieldFormat[thesis]{type}{#1\addcomma} I get this:
It removes my string definition for PhD and Master's.
\addcommaor\addperiodto the end of field formats should be a last resort. It is not done in any of the standard styles and may lead to unwanted output (though it is not at all unlikely that you will never get to see a problematic case - see also my 'matter of taste' comment under Alan's answer). – moewe May 01 '18 at 21:18natbib=trueis only required if you use thenatbib-like commands\citetand\citep, if you use thebiblatexnames\textciteand\parenciteinstead, you won't need that option. Instead of\defbibheading{bibliography}you could redefine the bibstringbibliography = {Bibliography}(depending on your classreferences = {Bibliography}might be needed as well). – moewe May 01 '18 at 21:23