How do I convert a querystring to a json string?

This gives the exactly same json you want

var dict = HttpUtility.ParseQueryString("ID=951357852456&FNAME=Jaime&LNAME=Lopez");
var json = new JavaScriptSerializer().Serialize(
                    dict.AllKeys.ToDictionary(k => k, k => dict[k])
           );

It also possible to use

var collection = HttpUtility.ParseQueryString(query);
Newtonsoft.Json.JsonConvert.SerializeObject(collection.AllKeys.ToDictionary(y => y, y => collection[y]));