Defender Declan Gallagher, 24, left Steven Findlay, 45, suffering from bleeding on the brain and a fractured skull after attacking him outside a hotel.
Mr Findlay had more than 22 staples inserted in his head and was kept in a hospital’s high dependency unit for five days after violence flared at The Parkville in Blantyre, Lanarkshire.
Former Celtic youth star Gallagher had been at the party with his family in April 2013, but told a jury at Hamilton Sheriff Court he had been caught up in trouble later in the evening.
He admitted he used his right hand to punch Mr Findlay to the ground after believing his father was going to be attacked.
But yesterday, Gallagher, of Dundee, shook his head as a jury convicted him of repeatedly punching and kicking Mr Findlay on the head and body, knocking him to the ground and striking him on the head with a baseball bat or similar implement to his severe injury and danger of life.
His co-accused, Anthony Murray, 29, of Blantyre, was also convicted of the same charge and a further charge of attacking Mr Findlay’s girlfriend Marie Dockery, 47.
Gallagher’s family sobbed uncontrollably as the verdicts were delivered.
Miss Dockery was also reduced to tears.
The week-long trial heard Gallagher claim fighting had started outside the hotel and he had left but later returned with his father and Murray when they encountered Mr Findlay.
He said a violent melee started but insisted he saw an unknown man use a bat to hit Mr Findlay.
The footballer added: “I didn’t have any weapons but I saw a man who I didn’t know with a baseball bat wearing dark clothing.
“I didn’t know there would be a weapon, there hadn’t been anything used before.
“The bat was about 2ft and he then struck Steven on the head with the bat. I saw him hit him twice.
“I did not hit anyone with a baseball bat.”
Mr Findlay had earlier told how he had lost his post as a head chef as he struggled to recover from his ordeal.
He said: “When I first looked in the mirror I saw two massive splits on the top of my head, heavy swelling on my face and a burst nose.
“I still get sore heads from time to time and my nasal passage is still blocked from where I was kicked.
“I lost my job after it because my boss didn’t take kindly to me being seriously injured.”
Sheriff Douglas Brown deferred sentence on the attackers until next month and called for background reports to consider.
Gallagher and Murray refused to comment as they left court yesterday.
A spokesman for Livingston FC said the club was aware of the verdict but refused to comment further.