The current world number 74, who has only ever reached one grand slam quarter-final, will kick off her Wimbledon campaign with serious momentum after winning her first singles title since the 2019 Nottingham Open.
Andreescu certainly didn't make things easy as she claimed the opening set by winning the tiebreak.
The former US Open champion also powered to a 4-2 lead in the second set, before Garcia rallied off four straight games to level the match.
Andreescu, who reached the final after Simona Halep was forced to withdraw from the semi-final due to a neck injury, went up 2-0 in the third and final set, but once again failed to take advantage of her positive start.
Instead, Garcia found another gear and claimed the set 6-4 to seal the title after a gruelling two hours and 42 minutes.
Garcia said in her post-match interview on court: "What a final it was, with a great fight every point from the first to the last.
"A grass court event is never easy the week before Wimbledon and is great for us players to feel like we have been here, a very special place, and I am really looking forward to coming back.
"When you win a title, you never know when is going to be the next one, it is what makes try to push your limits to be better.
"This means a lot because it has been a while, a long wait and I did it because I have a great team behind me."
Garcia's next game is in the opening round at Wimbledon as she takes on Britain's Yuriko Miyazaki.
Andreescu, on the other hand, will face American Emina Bektas in her first-round match at SW19.