How can I truncate a string using MVC Html Helpers?

Don't use the html helper. Just do this:

@item.Description.Substring(0, Math.Min(item.Description.Length, 25));

I'm assuming you are in some loop where item is the current element.

You could do this with an extension method.

public static string Truncate(this string source, int length)
{
    if (source.Length > length)
    {
        source = source.Substring(0, length);
    }

    return source;
}

Then in your view:

@item.Description.Truncate(25)

you could either truncate the data before it gets to the View, or use this Razor:

@{
    var shortDescript = String.Concat(modelItem.Take(25));
}
@Html.DisplayFor(modelItem => shortDescript)