Gareth Howles, who asked for this graphic picture showing his injuries to be released, was attacked as he walked through Fowlers Park in Wolverhampton when the robber approached.
The 23-year-old victim was on his way to care for his elderly grandfather and tried to flee, but was tripped and pinned to the ground.
When he refused to hand over the phone, the attacker lashed out several times with the blade before fleeing with a pair of earphones worth just a few pounds.
Mr Howles – who works for Wolverhampton young person's support group Base 25 – was left with multiple slash wounds to his face, including one cut that stretched across the bridge of his nose and narrowly missed his right eye.
Mr Howles, of Low Hill, was left needing 30 stitches.
He said: "It was an ordinary morning; I was walking through the park to take my disabled granddad on a shopping trip when a guy ran out of the bushes and squared up to me. I ran off but he clipped my ankle – I refused to hand over my phone or watch and in the end he just grabbed my earphones.
"It was only when I made my way out of the park and turned round to see a trail of blood did I realise I'd been cut – it happened very quickly and I guess adrenaline took over.
"I've been told I'll be scarred for life but I'm just grateful I still have my sight as the knife came very close to my right eye. I can't believe someone is prepared to use such violence and end up running off with a pair of earphones worth only a few pounds.
"It happened in broad daylight so I hope someone will have seen him running off in Fowlers Park and can give police information…or maybe someone has heard a person talking about the attack.
"This man needs locking up; if anyone has suspicions about who might be responsible then please call the police so we can get this man of the streets."
The robber – who was wearing a ski mask – struck at 9.40am on April 14 as Mr Howles was near the park's skating area at the entrance of Nine Elms Lane.
Dc Nick Taylor from Wolverhampton Police, said: "I need to hear from anyone who was in Fowlers Park on the morning of April 14 as they may have witnessed something important.
"We understand the attacker was a mixed-race male who was wearing black clothing and a ski mask. He emerged from bushes near the skate park; it could be that he'd been in the area for some time looking for potential victims. We're not sure what direction he ran off from after the attack."