JSON.NET: How to deserialize interface property based on parent (holder) object value?

You are on the right track. You do need to implement a custom JsonConverter for your Holder class to handle this situation, as you suggested. But, don’t worry, it is possible to write the converter in such a way that you can use the original reader and serializer instances passed to the converter, without ever needing to copy the settings over to new instances. Here is how I would write it:

class HolderConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return (objectType == typeof(Holder));
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JObject jo = JObject.Load(reader);
        Holder holder = new Holder();
        holder.ObjType = (int)jo["ObjType"];
        holder.Objects = new List<Base>();
        foreach (JObject obj in jo["Objects"])
        {
            if (holder.ObjType == 1)
                holder.Objects.Add(obj.ToObject<DerivedType1>(serializer));
            else
                holder.Objects.Add(obj.ToObject<DerivedType2>(serializer));
        }
        return holder;
    }

    public override bool CanWrite
    {
        get { return false; }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

Here is a quick demo:

class Program
{
    static void Main(string[] args)
    {
        string json = @"
        [
            {
                ""ObjType"" : 1,
                ""Objects"" : 
                [
                    { ""Id"" : 1, ""Foo"" : ""One"" },
                    { ""Id"" : 2, ""Foo"" : ""Two"" },
                ]
            },
            {
                ""ObjType"" : 2,
                ""Objects"" : 
                [
                    { ""Id"" : 3, ""Bar"" : ""Three"" },
                    { ""Id"" : 4, ""Bar"" : ""Four"" },
                ]
            },
        ]";

        List<Holder> list = JsonConvert.DeserializeObject<List<Holder>>(json);

        foreach (Holder holder in list)
        {
            if (holder.ObjType == 1)
            {
                foreach (DerivedType1 obj in holder.Objects)
                {
                    Console.WriteLine("Id: " + obj.Id + "  Foo: " + obj.Foo);
                }
            }
            else
            {
                foreach (DerivedType2 obj in holder.Objects)
                {
                    Console.WriteLine("Id: " + obj.Id + "  Bar: " + obj.Bar);
                }
            }
        }
    }
}

[JsonConverter(typeof(HolderConverter))]
class Holder
{
    public int ObjType { get; set; }
    public List<Base> Objects { get; set; }
}

abstract class Base
{
    public int Id { get; set; }
}

class DerivedType1 : Base
{
    public string Foo { get; set; }
}

class DerivedType2 : Base
{
    public string Bar { get; set; }
}

Output:

Id: 1  Foo: One
Id: 2  Foo: Two
Id: 3  Bar: Three
Id: 4  Bar: Four