I'd say Henry in arm-wrestling and Hafþór Júlíus Björnsson in a fight since he: a) has the reach and speed advantage, b) has been hanging out a lot recently at the WWE Fitness Center and may have gotten some insight into Henry's fighting style (professional wrestling), and c) because the fight would not be predetermined by a booking agent, and even if it were, I suspect that the Mountain would probably get a bigger push than the lukewarm 21st century of the World's Strongest Man.
@laflux, in Henry's prime, I'd take Sexual Chocolate over his later Hall of Pain, um, hall monitor? gimmick.
Mark Henry in his prime is a beast. I'l say he beats him both Rounds.
Even now, wrestlers have said in interviews that they are scared to wrestle him because they know he could easily tear their arm out of its socket (guys like Big Show & Orton have said that in shoot interviews).
The Mountain takes round 2 for sure, his sheer athleticism is far above Henry and his stamina dwarf Henry. As for arm wrestling... impossible to say since they banned arm wrestling from strongman competitions (with good reason)