How to make the last column in WPF datagrid take all the left space, always?

Set the width for the data grid to “Auto”. You’re allowing the columns to resize correctly within the grid itself, but you’ve hard-wired the width to 200.

UPDATE: Base on @micas’s comment, I may have misread. If that’s the case, try using 100 for the left column’s width and 100* for the right column (note the asterisk). This will default the width for the right column to 100 but allow it to resize to fill the grid.