class MainService : ServiceBase
{
  event EventHandler MyEvent;
  Thread m_Thread = null;
  ...
  protected override void OnStart(string[] args)
  {
    //тут System.Threading.Thread.CurrentThread.ManagedThreadId = 1
    MyEvent += delegate {
      //Тут System.Threading.Thread.CurrentThread.ManagedThreadId = 5
    };
    m_Thread = new Thread(new ThreadStart(MonitorThread));
    m_Thread.IsBackground = true;
    m_Thread.Start();
  }
  protected override void OnStop()
  {
    m_Thread.Abort();
  }
  ...
  private void MonitorThread()
  {
    //Something
    MyEvent?.Invoke(this, EventArg.Empty);
    //Something
  }
  ...
}