As he jailed him for 20 months, a judge said the fraud against 23 victims by a "brazen" and "persistent" liar was "one of the nastiest and meanest" he had ever encountered.
The betrayed groom, 30-year-old sports teacher Dino Carter, later said: "I was beyond devastated. The fact he could do that to everyone is beyond belief.
"I felt like I was living the script from a soap drama. It tore me apart that Martyn used the most important day of my life to get money.
"I'm mortified that the biggest conman I've ever known was the person I asked to be my best man."
Galvin was asked to be best man at his best friend's wedding before he had even proposed to his partner, said prosecutor Jenny Haigh.
He volunteered to arrange the two stag parties as it was the "best man's duty", contacted the groom's friends and set up a Facebook page.
They were meant to enjoy a three-day trip to Prague in August last year, followed by a trip to York Races in October.
As the nuptials grew closer, Galvin complained to Mr Carter that he was out of pocket from not getting money from the men.
He claimed he had tests and an operation for bowel cancer and needed radiotherapy.
When he did not attend meetings between the men, Galvin made excuses about hospital visits and "bad reactions" to medication.
As it was revealed there were no plane or hotel bookings on August 28, the groom still thought it was a "wind-up" or "mix-up".
When challenged, Galvin said he was terminally ill with cancer, while maintaining he had booked flights.
Galvin's mother broke the news to the shocked groom that his best friend did not have cancer as he claimed.
"I'm sorry," said Galvin when he was finally confronted days later.
Galvin, of Yarm, Stockton-on-Tees, said he was in debt and had planned to kill himself. He later admitted fraud by false representation between January and October 2015.
He had a 2008 conviction for a fraud in which he staged a robbery to steal £600 from his employer.
Duncan McReddie, defending, said Galvin was truly remorseful and he and his family had saved money to compensate the 23 victims in full.
He told how Galvin had a long-standing gambling addiction, which he put above the needs of others.
Mr McReddie added that Galvin did "flash the cash", but sank deeper and deeper into debt as he chased the "next big win". Galvin has since received help for the gambling addiction, he added.
Mr McReddie said the cancer claims were lies, but Galvin did have a "chronic digestive problem".
Judge Simon Bourne-Arton QC, the Recorder of Middlesbrough, told Galvin: "This fraud is perhaps one of the nastiest and meanest I've encountered in my time involved in criminal law.
"The fraud involved a string of quite dreadful lies, lies to your best friend, a man who entrusted in you the task and the honour of being his best man.
"To lie as you did so brazenly, so persistently and in such detail about your health, only you know how you could do that. You extravagantly were spending, flashing the cash on yourself."
He jailed Galvin for 20 months and ordered him to pay the full compensation in 28 days.
