json.net does not serialize properties from derived class

Adding the attribute [JsonObject(MemberSerialization.OptOut)] to your derived class will include all its public members to be serialized.

[Table(Name = "dbo.mytable")]
[JsonObject(MemberSerialization.OptOut)]
public sealed class mytable : DataEntity
{
    ...
}

Alternatively, if you only want certain properties of your derived class to be serialized you can add the attribute [JsonProperty] to each one (This would be equivalent of adding [DataMember] to each property along with [DataContract] on the class).

Yes, you are missing the [DataContract] attribute on the derived class. You also need to add [DataMember] to any properties or fields that you want serialized, if you haven't already added them. Json.Net was changed in version 5.0 release 1 (April 2013) such that the [DataContract] attribute is not inherited.

Note that if you remove all instances of [DataContract] and [DataMemeber] from your classes, Json.Net behaves differently: in that case, the default behavior is for Json.Net to serialize all public properties, both in the base and derived classes.