Ranked number 38 in the world two years ago, the 22-year-old admitted she had long suffered from depression and was no longer willing to make the sacrifices necessary to reach the top.
"After thinking long and hard, I do not have the passion or enjoyment to drive myself to the level I would like to be at in professional tennis," Marino said.
"I have previously been number 38 on the WTA singles rankings so I realise the amount of work and sacrifices that have to be put in and at this point I do not think it is worth sacrificing my happiness.
"I feel there is more to life than just tennis."
Big and powerful, Marino appeared destined for a promising career on the WTA circuit and under-scored her potential in a hard fought second round clash with Venus Williams at the 2010 US Open.
"Now I know what it's like to play myself," Williams said after winning the match 7-6 6-3.
The following season, Marino backed up that potential by reaching her first career WTA final in Memphis and advancing to the third round of the French Open.
But the climb up the rankings had already begun to take its toll on the Canadian and in February 2012 she took a seven month break from the sport citing mental and physical fatigue.
Compounding her depression were cruel attacks on Twitter and social media, some coming from angry gamblers who had lost money backing her, while others poked fun at her appearance and weight.
"My depression came way before the so called cyber-bullying; this has been going on for I would dare say six years," said Marino.
"I was getting some comments that were really hurtful, as well as some great comments, but the hurtful ones stick with you a bit more.
"I was getting messages that I should die, that I should go burn in hell, that I'm a dumb ass, an idiot, that I lost them money, a wide variety of things and that is just scratching the surface.
"I like to feel I have a thick skin and I can deal with these sorts of things so it wasn't the main factor.
"Social media has taken its toll on me but it is not the main reason ... the reason I am stepping back is that I don't think I'm willing to sacrifice my happiness and other parts of my life to tennis."
After her extended break, Marino returned to the tour last September, winning a Challenger event and then playing at the Australian Open in January.
Her passion for the sport did not return as quickly as her form, however.
"After the Australian Open when I came back into tournaments I just didn't find that spark in myself," said Marino.
"I thought maybe it would come back to me but the more I started playing the more I found it was becoming more-and-more apparent I didn't have the passion it would take."
Marino says the focus of her life will now be on going back to school, finding a job, spending time with friends and family and enjoying a normal life.
Although she refused to call her decision a retirement, Marino's tone and words indicated there was little chance of a return.
Marino also doubted she would ever return to social media after shutting down her Twitter and Facebook accounts on Tuesday before making her announcement.
"At this point I can't really see myself going back onto social media but who knows," said Marino. "I don't really find it something I really need in my life at this moment.
"In a way I wish I hadn't joined social media because of the criticism I received but I can't really go about regretting what I've done."
- Sports & Recreation