Marine Joyce only found out his father would be giving him his beret halfway through the "hideous" 30-mile walk: "I was high on adrenaline and high on morale for that.
"[I’m] over the moon and to have my dad here is the best feeling."
Warrant Officer Joyce works as a Royal Marines welfare specialist worker and told Forces News it was a "very, very proud" day for him.
"It was a shock when he first told me he was going to follow in my footsteps and join the Royal Marines," said WO2 Joyce.
"[There were] mixed emotions about that having done 27 years myself and experienced any highs and lows but immensely proud to come down here today and be part of this for him.
“I know what it’s like to be stood there, I know how tough it is to get through training."
Prince Harry's visit to the event was kept under wraps and personnel were only told that a 'VIP' would be in attendance.
As Marines ran over a bridge to finish off the 30-mile Commando Test on Dartmoor they were treated to a round of applause from the watching crowd and a salute from their Captain General, the Duke of Sussex.
Both Joyce father and son got to meet Prince Harry afterwards, something Marine Joyce described as "another surreal feeling to just top a crazy day".