Don't do it like that: all you end up with is a string you can't use:
d:\temp\myfile.txtc:\folder\myotehrfile.txt
Instead, use the String.Join method:
string toSave = string.Join("|", MRUList);
File.WriteAllText(pathToFile, toSave);
Then to read them back:
string rawData = File.ReadAllText(pathToFile);
string[] individualFiles = rawData.Split('|');
Since the '|' character can;t appear in any Windows file or folder name it's safe to use as a separator.
And BTW: don't store data in the application folder. It works in development, but tends to fail in production. See here:
Where should I store my data?[
^] it shows some better places and how to use them.