IQ is well correlated with job performance, so if you think the potential employer cares mainly about future performance (and knows what he or she is doing) include your IQ and that it qualifies you for Mensa. However, I suppose that most people who review applications would see a reference to IQ/Mensa membership as a downside. Mensans manage to be seen as both elitist and low status at the same time. They are usually smart people (mostly smart men actually) who have lower status jobs, so I doubt they would be very useful professional connections.

As to whether joining Mensa might have intrinsic value to you, I don't know. I think they mainly trade puzzles with each other.