Biblatex --- how to add comma before and between two names

Question

I'm currently getting:

Surname, IN and Secondauthorsurname, FM.

I need:

Surname, IN, and Secondauthorsurname, FM.

Searching for "comma" in the documentation is impossible, since it also hits on "command"

\documentclass{memoir}

\usepackage[american]{babel}

\usepackage{biblatex}
\DeclareLanguageMapping{american}{american}

\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@Article{Reference:1994,
  author =   {First I. Last, and Second Y. Author},
  title =    {This is the article title},
  journal =  {T Journal T},
  journallongtitle =     {The Journal Title},
  year =     1994,
  volume =   50,
  pages =    {30--40}
}
}
\end{filecontents}

\addbibresource{\jobname.bib}

\nocite{*}

\begin{document}

\printbibliography

\end{document}

Answer 1

Try the following redefinitions in your preamble.

\renewcommand*{\finalnamedelim}{%
  \finalandcomma
  \addspace\bibstring{and}\space}

\renewcommand*{\finallistdelim}{%
  \finalandcomma
  \addspace\bibstring{and}\space}

The MWE

\documentclass{memoir}
\usepackage[american]{babel}
\usepackage{biblatex}

\begin{filecontents}{\jobname.bib}
@Article{Reference:1994,
  author =   {First Last and Second Author},
  title =    {This is the article title},
  journal =  {T Journal T},
  year =     1994,
  volume =   50,
  pages =    {30--40}
}
}
\end{filecontents}

\renewcommand*{\finalnamedelim}{%
  \finalandcomma
  \addspace\bibstring{and}\space}

\renewcommand*{\finallistdelim}{%
  \finalandcomma
  \addspace\bibstring{and}\space}

\addbibresource{\jobname.bib}

\begin{document}
  \nocite{*}
  \printbibliography
\end{document}

yields